Exploration routing chapter 01 11 full

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1

Introduction to Routing and Packet Forwarding

Routing Protocols and Concepts – Chapter 1

2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Objectives Identify a router as a computer with an OS and

hardware designed for the routing process.

Demonstrate the ability to configure devices and apply addresses.

Describe the structure of a routing table.

Describe how a router determines a path and switches packets


Router as a Computer Describe the basic purpose of a router

-Computers that specialize in sending packets over the data network. They are responsible for interconnecting networks by selecting the best path for a packet to travel and forwarding packets to their destination

Routers are the network center

-Routers generally have 2 connections:

-WAN connection (Connection to ISP)

-LAN connection


Router as a Computer Data is sent in front of packets between 2 end devices

Routers are used to direct packet to its destination


Router as a Computer Routers examine a packet’s destination IP address and

determine the best path by enlisting the aid of a routing table


Router as a Computer Router components and their functions”

CPU - Executes operating system instructions

Random access memory (RAM) - Contains the running copy of configuration file. Stores routing table. RAM contents lost when power is off

Read-only memory (ROM) - Holds diagnostic software used when router is powered up. Stores the router’s bootstrap program.

Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP addresses (Routing protocol, Hostname of router)

Flash memory - Contains the operating system (Cisco IOS)

Interfaces - There exist multiple physical interfaces that are used to connect network. Examples of interface types:

-Ethernet / fast Ethernet interfaces

-Serial interfaces

-Management interfaces


Router as a Computer Router components


Router as a Computer Major phases to the

router boot-up process

Test router hardware

Power-On Self Test (POST)

Execute bootstrap loader

Locate & load Cisco IOS software

-Locate IOS

-Load IOS

Locate & load startup configuration file or enter setup mode

-Bootstrap program looks for configuration file


Router as a Computer Verify the router boot-up process:

-The show version command is used to view information about the router during the bootup process. Information includes:

Platform model number

Image name & IOS version

Bootstrap version stored in ROM

Image file name & where it was loaded from

Number & type of interfaces

Amount of NVRAM

Amount of flash

Configuration register


Router as a Computer


Router as a Computer Router Interface is a physical connector that enables a

router to send or receive packets

Each interface connects to a separate network

Consist of socket or jack found on the outside of a router

Types of router interfaces:

-Ethernet

-Fastethernet

-Serial

-DSL

-ISDN

-Cable


Router as a Computer Two major groups of Router Interfaces

LAN Interfaces:

Are used to connect router to LAN network

Has a layer 2 MAC address

Can be assigned a Layer 3 IP address

Usually consist of an RJ-45 jack

WAN Interfaces

Are used to connect routers to external networks that interconnect LANs.

Depending on the WAN technology, a layer 2 address may be used.

Uses a layer 3 IP address


Router as a Computer Routers and the Network Layer

Routers use destination IP address to forward packets

The path a packet takes is determined after a router consults information in the routing table.

After router determines the best path

Packet is encapsulated into a frame

Frame is then placed on network medium in form of Bits


Router as a Computer Routers Operate at Layers 1, 2 & 3

Router receives a stream of encoded bits

Bits are decoded and passed to layer 2

Router de-encapsulates the frame

Remaining packet passed up to layer 3

-Routing decision made at this layer by examining destination IP address

Packet is then re-encapsulated & sent out outbound interface


Configure Devices and Apply Addresses Implementing Basic Addressing Schemes

When designing a new network or mapping an existing network you must provide the following information in the form of a document:

-Topology drawing that Illustrates physical connectivity

–Address table that provides the following information:

Device name

Interfaces used

IP addresses

Default gateway


Configure Devices and Apply Addresses Basic Router Configuration

A basic router configuration should contain the following:

-Router name - Host name should be unique

-Banner - At a minimum, banner should warn against unauthorized use

-Passwords - Use strong passwords

-Interface configurations - Specify interface type, IP address and subnet mask. Describe purpose of interface. Issue no shutdown command. If DCE serial interface issue clock rate command.

After entering in the basic configuration the following tasks should be completed

-Verify basic configuration and router operations.

-Save the changes on a router


Configure Devices and Apply Addresses


Configure Devices and Apply Addresses Verify Basic Router Configuration

-Issue the show running-config command

-Save the basic router configuration by Issuing the copy running-config startup-config command

-Additional commands that will enable you to further verify router configuration are:

Show running-config - Displays configuration currently in RAM

Show startup-config - Displays configuration file NVRAM

Show IP route - Displays routing table

Show interfaces - Displays all interface configurations

Show IP int brief - Displays abbreviated interface configuration information


Routing Table Structure Routing Table is stored in ram and contains information

about:

Directly connected networks - this occurs when a device is connected to another router interface

Remotely connected networks - this is a network that is not directly connected to a particular router

Detailed information about the networks include source of information, network address & subnet mask, and Ip address of next-hop router

Show ip route command is used to view a routing table


Routing Table Structure Adding a connected network to the routing table

-Router interfaces

Each router interface is a member of a different network

Activated using the no shutdown command

In order for static and dynamic routes to exist in routing table you must have directly connected networks


Routing Table Structure Static routes in the routing table

-Includes: network address and subnet mask and IP address of next hop router or exit interface

-Denoted with the code S in the routing table

-Routing tables must contain directly connected networks used to connect remote networks before static or dynamic routing can be used

When to use static routes

-When network only consists of a few routers

-Network is connected to internet only through one ISP

-Hub & spoke topology is used on a large network


Routing Table Structure Connected and Static routes


Routing Table Structure Dynamic routing protocols

-Used to add remote networks to a routing table

-Are used to discover networks

-Are used to update and maintain routing tables

Automatic network discovery

-Routers are able discover new networks by sharing routing table information


Routing Table Structure Maintaining routing tables

-Dynamic routing protocols are used to share routing information with other router & to maintain and up date their own routing table.

IP routing protocols. Example of routing protocols include:

-RIP

-IGRP

-EIGRP

-OSPF


Routing Table Structure Routing Table Principles

-3 principles regarding routing tables:

Every router makes its decisions alone, based on the information it has in its routing table.

Different routing table may contain different information

A routing table can tell how to get to a destination but not how to get back


Routing Table Structure Effects of the 3 Routing Table Principles

-Packets are forwarded through the network from one router to another, on a hop by hop basis.

-Packets can take path “X” to a destination but return via path “Y” (Asymmetric routing).


Router Paths and Packet Switching Internet Protocol (IP) packet format contains fields that

provide information about the packet and the sending and receiving hosts

Fields that are importance for CCNA students:

-Destination IP address

-Source IP address

-Version & TTL

-IP header length

-Precedence & type of service

-Packet length


Router Paths and Packet Switching MAC Layer Frame Format

MAC Frames are also divided into fields. They include:

-Preamble

-Start of frame delimiter

-Destination MAC address

-Source MAC address

-Type/length

-Data and pad

-Frame check sequence


Router Paths and Packet Switching A Metric is a numerical value used by routing protocols help

determine the best path to a destination

–The smaller the metric value the better the path

2 types of metrics used by routing protocols are:

-Hop count - this is the number of routers a packet must travel through to get to its destination

-Bandwidth - this is the “speed” of a link also known as the data capacity of a link


Router Paths and Packet Switching Equal cost metric is a condition where a router has multiple paths

to the same destination that all have the same metric

To solve this dilemma, a router will use Equal Cost Load Balancing. This means the router sends packets over the multiple exit interfaces listed in the routing table.


Router Paths and Packet Switching Path determination is a process used by a router to pick the best

path to a destination

One of 3 path determinations results from searching for the best path

Directly connected network

Remote network

No route determined


Router Paths and Packet Switching Switching Function of Router is the process used by a

router to switch a packet from an incoming interface to an outgoing interface on the same router.

-A packet received by a router will do the following:

Strips off layer 2 headers.

Examines destination IP address located in Layer 3 header to find best route to destination.

Re-encapsulates layer 3 packet into layer 2 frame.

Forwards frame out exit interface.


Router Paths and Packet Switching As a packet travels from one networking device to another

-The Source and Destination IP addresses NEVER change

-The Source & Destination MAC addresses CHANGE as packet is forwarded from one router to the next.

-TTL field decrement by one until a value of zero is reached at which point router discards packet (prevents packets from endlessly traversing the network)


Router Paths and Packet Switching Path determination and switching function details. PC1

Wants to send something to PC 2 here is part of what happens

Step 1 - PC1 encapsulates packet into a frame. Frame contains R1’s destination MAC address


Router Paths and Packet Switching

Step 2 - R1 receives Ethernet frame.

R1 sees that destination MAC address matches its own MAC.

R1 then strips off Ethernet frame.

R1 Examines destination IP.

R1 consults routing table looking for destination IP.

After finding destination IP in routing table, R1 now looks up next hop IP address.

R1 re-encapsulates IP packet with a new Ethernet frame.

R1 forwards Ethernet packet out Fa0/1 interface.


Router Paths and Packet Switching


Router Paths and Packet Switching Path determination and switching function details. PC1 Wants to

send something to PC 2 here is part of what happens

Step 3 - Packet arrives at R2

R2 receives Ethernet frame

R2 sees that destination MAC address matches its own MAC

R2 then strips off Ethernet frame

R2 Examines destination IP

R2 consults routing table looking for destination IP

After finding destination IP in routing table, R2 now looks up next hop IP address

R2 re-encapsulates IP packet with a new data link frame

R2 forwards Ethernet packet out S0/0 interface


Router Paths and Packet Switching Path determination and switching function details. PC1 Wants to

send something to PC 2 here is part of what happens

Step 4 - Packet arrives at R3

R3 receives PPP frame

R3 then strips off PPP frame

R3 Examines destination IP

R3 consults routing table looking for destination IP

After finding destination IP in routing table, R3 is directly connected to destination via its fast Ethernet interface

R3 re-encapsulates IP packet with a new Ethernet frame

R3 forwards Ethernet packet out Fa0/0 interface

Step 5 - IP packet arrives at PC2. Frame is decapsulated & processed by upper layer protocols.


Summary

Routers are computers that specialize in sending data over a network.

Routers are composed of:

-Hardware i.e. CPU, Memory, System bus, Interfaces

-Software used to direct the routing process

IOS

Configuration file

Routers need to be configured. Basic configuration consists of:

-Router name

-Router banner

-Password(s)

-Interface configurations i.e. IP address and subnet mask

Routing tables contain the following information

-Directly connected networks

-Remotely connected networks

-Network addresses and subnet masks

-IP address of next hop address


Summary Routers determine a packets path to its destination by

doing the following

Receiving an encapsulated frame & examining destination MAC address.

If the MAC address matches then Frame is de-encapsulated so that router can examine the destination IP address.

If destination IP address is in routing table or there is a static route then Router determines next hop IP address. Router will re-encapsulate packet with appropriate layer 2 frame and send it out to next destination.

Process continues until packet reaches destination.

Note - only the MAC addresses will change the source and destination IP addresses do not change.



Static Routing



Objectives Define the general role a router plays in networks.

Describe the directly connected networks, different router interfaces

Examine directly connected networks in the routing table and use the CDP protocol (Cisco Discovery Protocol)

Describe static routes with exit interfaces

Describe summary and default route

Examine how packets get forwarded when using static routes

Identify how to manage and troubleshoot static routes

http://en.wikipedia.org/wiki/Cisco_Discovery_Protocol


General Role of the Router Functions of a Router

Best Path Selections

Forwarding packets to destination

Introducing the Topology

3 1800 series routers connected via WAN links

Each router connected to a LAN represented by a switch and a PC


General Role of the Router Connections of a Router for WAN

-A router has a DB-60 port that can support 5 different cabling standards

Connections of a Router for Ethernet

-2 types of connectors can be used: Straight through and Cross-over

Straight through used to connect:

-Switch-to-Router, Switch-to-PC, Router-to-Server, Hub-to-PC, Hub-to-Server

Cross-over used to connect:

-Switch-to-Switch, PC-to-PC, Switch-to-Hub, Hub-to-Hub, Router-to-Router


Interfaces Examining Router Interfaces

-Show IP router command – used to view routing table

-Show Interfaces command – used to show status of an interface

-Show IP Interface brief command – used to show a portion of the interface information

-Show running-config command – used to show configuration file in RAM


Interfaces Configuring an Ethernet interface

-By default all serial and Ethernet interfaces are down

-To enable an interface use the No Shutdown command


Interfaces Verifying Ethernet interface

-Show interfaces for fastEthernet 0/0 – command used to show status of fast Ethernet port

-Show ip interface brief

-Show running-config

Ethernet interfaces participate in ARP


Interfaces Configuring a Serial interface

-Enter interface configuration mode

-Enter in the ip address and subnet mask

-Enter in the no shutdown command

Example:

-R1(config)#interface serial 0/0

-R1(config-if)#ip address 172.16.2.1 255.255.255.0

-R1(config-if)#no shutdown


Interfaces Examining Router Interfaces

-Physically connecting a WAN Interface.

-A WAN Physical Layer connection has sides:

Data Circuit-terminating Equipment (DCE) – This is the service provider. CSU/DSU is a DCE device.

Data Terminal Equipment (DTE) – Typically the router is the DTE device.


Interfaces Configuring serial links in a lab environment

One side of a serial connection must be considered a DCE

This requires placing a clocking signal – use the clock rate command.

Example:

-R1(config)#interface serial 0/0

-R1(config-if)#clockrate 64000

Serial Interfaces require a clock signal to control the timing of the communcations.


Routing Table and CDP Protocol Purpose of the debug ip routing command

Allows you to view changes that the router performs when adding or removing routes

Example:

-R2#debug ip routing

-IP routing debugging is on


Routing Table and CDP Protocol To configure an Ethernet interface

Example:

-R2(config)#interface fastethernet 0/0

-R2(config-if)#ip address 172.16.1.1 255.255.255.0

-R2(config-if)#no shutdown


Routing Table and CDP Protocol When a router only has its interfaces configured & no

other routing protocols are configured then:

-The routing table contains only the directly connected networks

-Only devices on the directly connected networks are reachable


Routing Table and CDP Protocol


Routing Table and CDP Protocol

Checking each route in turn

The pingcommand is used to check end to end connectivity


Routing Table and CDP Protocol Purpose of CDP

A layer 2 cisco proprietary tool used to gather information about other directly connected Cisco devices.

Concept of neighbors

-2 types of neighbors

Layer 3 neighbors

Layer 2 neighbors


Routing Table and CDP Protocol CDP show commands

Show cdp neighbors command

-Displays the following information:

Neighbor device ID

Local interface

Holdtime value, in seconds

Neighbor device capability code

Neighbor hardware platform

Neighbor remote port ID

Show cdp neighbors detail command

-Useful in determining if an IP address configuration error


Routing Table and CDP Protocol Disabling CDP

To disable CDP globally use the following command

Router(config)#no cdp run


Static Routes with Exit Interfaces Purpose of a static route

A manually configured route used when routing from a network to a stub network


Static Routes with Exit Interfaces IP route command

To configure a static route use the following command: ip route

Example:

-Router(config)# ip route network-address subnet-mask {ip-address | exit-interface }


Static Routes with Exit Interfaces Dissecting static route syntax

ip route - Static route command

172.16.1.0 – Destination network address

255.255.255.0 - Subnet mask of destination network

172.16.2.2 - Serial 0/0/0 interface IP address on R2, which is the "next-hop" to this network


Static Routes with Exit Interfaces Configuring routes to 2 or more remote networks

Use the following commands for R1

-R1(config)#ip route 192.168.1.0 255.255.255.0 172.16.2.2

-R1(config)#ip route 192.168.2.0 255.255.255.0 172.16.2.2


Static Routes with Exit Interfaces Zinin’s 3 routing principles

Principle 1: "Every router makes its decision alone, based on the information it has in its own routing table.“

Principle 2: "The fact that one router has certain information in its routing table does not mean that other routers have the same information.“

Principle 3: "Routing information about a path from one network to another does not provide routing information about the reverse, or return path."


Static Routes with Exit Interfaces Using Zinin’s 3 routing principles, how would you

answer the following?

-Would packets from PC1 reach their destination?

Yes, packets destined for 172.16.1.0/24 and 192.168.1.0/24 networks would reach their destination.

-Does this mean that any packets from these networks destined for 172.16.3.0/24 network will reach their destination?

No, because neither R2 nor R3 router has a route to the 172.16.3.0/24 network.


Static Routes with Exit Interfaces Resolving to an Exit Interface

-Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet. A static route that forwards all packets to the next-hop IP address goes through the following process (reclusive route lookup)

The router first must match static route’s destination IP address with the Next hop address

The next hop address is then matched to an exit interface


Static Routes with Exit Interfaces Configuring a Static route with an Exit Interface

-Static routes configured with an exit interface are more efficientbecause the routing

–The routing table can resolve the exit interface in a single search instead of 2 searches

-Example of syntax require to configure a static route with an exit interface


Static Routes with Exit Interfaces Modifying Static routes

Existing static routes cannot be modified. The old static route must be deleted by placing no in front of the ip route

Example:

-no ip route 192.168.2.0 255.255.255.0 172.16.2.2

A new static route must be rewritten in the configuration


Static Routes with Exit Interfaces Verifying the Static Route Configuration

-Use the following commands

Step 1 show running-config

Step 2 verify static route has been entered correctly

Step 3 show ip route

Step 4 verify route was configured in routing table

Step 5 issue ping command to verify packets can reach destination and that Return path is working


Static Routes with Exit Interfaces Ethernet interfaces and ARP.

– If a static route is configured on an Ethernet link

-If the packet is sent to the next-hop router then…

the destination MAC address will be the address of the next hop’s Ethernet interface

This is found by the router consulting the ARP table.

If an entry isn’t found then an ARP request will be sent out


Summary and Default Route Summarizing routes reduces the size of the routing

table.

Route summarization is the process of combining a number of static routes into a single static route.


Summary and Default Route Configuring a summary route

Step 1: Delete the current static route

Step 2: Configure the summary static route

Step 3: Verify the new static route


Summary and Default Route Default Static Route

This is a route that will match all packets. Stub routers that have a number of static routes all exiting the same interface are good candidates for a default route.

-Like route summarization this will help reduce the size of the routing table

Configuring a default static route

Similar to configuring a static route. Except that destination IP address and subnet mask are all zeros

Example:

-Router(config)#ip route 0.0.0.0 0.0.0.0 [exit-interface | ip-address ]


Summary and Default Route Static routes and subnet masks

The routing table lookup process will use the most specific match when comparing destination IP address and subnet mask

Default static routes and subnet masks

Since the subnet mask used on a default static route is 0.0.0.0 all packets will match.


Static Routes and Packet Forwarding

Packet forwarding with static routes. (recall Zinin’s 3 routing principles)

Router 1

Packet arrives on R1’s Fastethernet 0/0 interface

R1 does not have a route to the

destination network, 192.168.2.0/24

R1 uses the default

static route.


Static Routes and Packet Forwarding Packet forwarding with static routes. (recall Zinin’s 3

routing principles)

Router 2

The packet arrives on the Serial 0/0/0 interface on R2.

R2 has a static route to 192.168.2.0/24 out Serial0/0/1.


Static Routes and Packet Forwarding Packet forwarding with static routes. (recall Zinin’s 3

routing principles)

Router 3

The packet arrives on the Serial0/0/1 interface on R3.

R3 has a connected route to 192.168.2.0/24 out Fastethernet 0/1.


Static Routes and Packet Forwarding Troubleshooting a Missing Route

Tools that can be used to isolate routing problems include:

-Ping– tests end to end connectivity

-Traceroute– used to discover all of the hops (routers) along the path between 2 points

-Show IP route– used to display routing table & ascertain forwarding process

-Show ip interface brief- used to show status of router interfaces

-Show cdp neighbors detail– used to gather configuration information about directly connected neighbors


Static Routes and Packet Forwarding Solving a Missing Route

Finding a missing or mis-configured route requires methodically using the correct tools

-Start with PING. If ping fails then use traceroute to determine where packets are failing to arrive

Issue: show ip route to examine routing table.

-If there is a problem with a mis-configured static route remove the static route then reconfigure the new static route






Summary Routers

-Operate at layer 3

-Functions include best path selection & forwarding packets

Connecting Networks

WANs

Serial cables are connected to router serial ports.

In the lab environment clock rates must be configured for DCE

LANs

Straight through cables or cross over cables are used to connect to fastethernet port. (The type of cable used depends on what devices are being connected)

Cisco Discovery Protocol

A layer 2 proprietary protocol

Used to discover information about directly connected Cisco devices


Summary Static Routes

-This is a manually configured path that specifies how the router will get to a certain point using a certain path.

Summary static routes

-This is several static routes that have been condensed into a single static route.

Default route

-It is the route packets use if there is no other possible match for their destination in the routing table.

Forwarding of packets when static route is used

-Zinin’s 3 routing principles describe how packets are forwarded

Troubleshooting static routes may require some of the following commands:

-Ping

-Traceroute

-Show IP route

-Show ip interface brief

-Show cdp neighbors detail



Introduction to Dynamic Routing Protocol



Objectives Describe the role of dynamic routing protocols and

place these protocols in the context of modern network design.

Identify several ways to classify routing protocols.

Describe how metrics are used by routing protocols and identify the metric types used by dynamic routing protocols.

Determine the administrative distance of a route and describe its importance in the routing process.

Identify the different elements of the routing table.


Dynamic Routing Protocols Function(s) of Dynamic Routing Protocols:

-Dynamically share information between routers.

-Automatically update routing table when topology changes.

-Determine best path to a destination.


Dynamic Routing Protocols The purpose of a dynamic routing protocol is to:

-Discover remote networks

-Maintaining up-to-date routing information

-Choosing the best path to destination networks

-Ability to find a new best path if the current path is no longer available


Dynamic Routing Protocols Components of a routing protocol

Algorithm

In the case of a routing protocol algorithms are used for facilitating routing information and best path determination

Routing protocol messages

These are messages for discovering neighbors and exchange of routing information


Dynamic Routing Protocols Advantages of static routing

-It can backup multiple interfaces/networks on a router

-Easy to configure

-No extra resources are needed

-More secure

Disadvantages of static routing

-Network changes require manual reconfiguration

-Does not scale well in large topologies


Classifying Routing Protocols Dynamic routing protocols are grouped according to

characteristics. Examples include:

-RIP

-IGRP

-EIGRP

-OSPF

-IS-IS

-BGP

Autonomous System is a group of routers under the control of a single authority.


Classifying Routing Protocols Types of routing protocols:

-Interior Gateway Protocols (IGP)

-Exterior Gateway Protocols (EGP)


Classifying Routing Protocols Interior Gateway Routing Protocols (IGP)

-Used for routing inside an autonomous system & used to route within the individual networks themselves.

-Examples: RIP, EIGRP, OSPF

Exterior Routing Protocols (EGP)

-Used for routing between autonomous systems

-Example: BGPv4


Classifying Routing Protocols IGP: Comparison of Distance Vector & Link State

Routing Protocols

Distance vector

– routes are advertised as vectors

of distance & direction.

– incomplete view of network

topology.

–Generally, periodic

updates.

Link state

– complete view of network

topology is created.

– updates are not

periodic.


Classifying Routing Protocols


Classifying Routing Protocols

Classful routing protocols

Do NOT send subnet mask in routing updates

Classless routing protocols

Do send subnet mask in

routing updates.


Classifying Routing Protocols Convergence is defined as when all routers’ routing

tables are at a state of consistency


Routing Protocols Metrics Metric

A value used by a routing protocol to determine which routes are better than others.


Routing Protocols Metrics Metrics used in IP routing protocols

-Bandwidth

-Cost

-Delay

-Hop count

-Load

-Reliability


Routing Protocols Metrics The Metric Field in the

Routing Table

Metric used for each routing protocol

-RIP - hop count

-IGRP & EIGRP -Bandwidth (used by default), Delay (used by default), Load, Reliability

-IS-IS & OSPF – Cost, Bandwidth (Cisco’s implementation)


Routing Protocols Metrics Load balancing

This is the ability of a router to distribute packets among multiple same cost paths


Administrative Distance of a Route Purpose of a metric

It’s a calculated value used to determine the best path to a destination

Purpose of Administrative Distance

It’s a numeric value that specifies the preference of a particular route


Administrative Distance of a Route Identifying the Administrative Distance (AD) in a

routing table

It is the first number in the brackets in the routing table


Administrative Distance of a Route Dynamic Routing Protocols


Administrative Distance of a Route Directly connected routes

Have a default AD of 0

Static Routes

Administrative distance of a static route has a default value of 1


Administrative Distance of a Route Directly connected routes

-Immediately appear in the routing table as soon as the interface is configured


Summary Dynamic routing protocols fulfill the following functions

-Dynamically share information between routers

-Automatically update routing table when topology changes

-Determine best path to a destination

Routing protocols are grouped as either

-Interior gateway protocols (IGP)Or

-Exterior gateway protocols(EGP)

Types of IGPs include

-Classless routing protocols - these protocols include subnet mask in routing updates

-Classful routing protocols - these protocols do not include subnet mask in routing update


Summary

Metrics are used by dynamic routing protocols to calculate the best path to a destination.

Administrative distance is an integer value that is used to indicate a router’s “trustworthiness”

Components of a routing table include:

-Route source

-Administrative distance

-Metric


© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

ITE PC v4.0

Chapter 1 110

Distance Vector Routing Protocols


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Chapter 1 111© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Objectives Identify the characteristics of distance vector routing

protocols.

Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP).

Describe the processes to maintain accurate routing tables used by distance vector routing protocols.

Identify the conditions leading to a routing loop and explain the implications for router performance.

Recognize that distance vector routing protocols are in use today

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Examples of Distance Vector routing protocols:

Routing Information Protocol (RIP)

Interior Gateway Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol

(EIGRP)

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Distance Vector Routing Protocols Distance Vector Technology

–The Meaning of Distance Vector:

•A router using distance vector routing protocols knows 2 things:

Distance to final destination

Vector, or direction, traffic should be directed

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Characteristics of Distance Vector routing protocols:

Periodic updates

Neighbors

Broadcast updates

Entire routing table is included with routing update

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Distance Vector Routing Protocols Routing Protocol Algorithm:

-Defined as a procedure for accomplishing a certain task

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Routing Protocol Characteristics

–Criteria used to compare routing protocols includes

-Time to convergence

-Scalability

-Resource usage

-Implementation & maintenance

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Network Discovery Router initial start up (Cold Starts)

-Initial network discovery

Directly connected networks are initially placed in routing table

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Network Discovery

Initial Exchange of Routing Information

–If a routing protocol is configured then

-Routers will exchange routing information

Routing updates received from other routers

-Router checks update for new information

If there is new information:

-Metric is updated

-New information is

stored in routing table

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Network Discovery Exchange of Routing Information

–Router convergence is reached when

-All routing tables in the network contain the same network information

–Routers continue to exchange routing information

-If no new information is found then Convergence is reached

ITE PC v4.0


Network Discovery

Convergence must be reached before a network is considered completely operable

Speed of achieving convergence consists of 2 interdependent categories

-Speed of broadcasting routing information

-Speed of calculating routes

ITE PC v4.0


Routing Table Maintenance

Periodic Updates: RIPv1 & RIPv2

These are time intervals in which a router sends out its entire routing table.

ITE PC v4.0



RIP uses 4 timers

-Update timer

-Invalid timer

-Holddown timer

-Flush timer

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Bounded Updates: EIGRP

EIRPG routing updates are

-Partial updates

-Triggered by topology changes

-Bounded

-Non periodic

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Triggered Updates

–Conditions in which triggered updates are sent

-Interface changes state

-Route becomes unreachable

-Route is placed in routing table

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Random Jitter

Synchronized updates

A condition where multiple routers on multi access LAN segments transmit routing updates at the same time.

Problems with synchronized updates

-Bandwidth consumption

-Packet collisions

Solution to problems with

synchronized updates

- Used of random variable

called RIP_JITTER

ITE PC v4.0


Routing Loops

Routing loops are

A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.

ITE PC v4.0


Routing Loops

Routing loops may be caused by:

-Incorrectly configured static routes

-Incorrectly configured route redistribution

-Slow convergence

-Incorrectly configured discard routes

Routing loops can create the following issues

-Excess use of bandwidth

-CPU resources may be strained

-Network convergence is degraded

-Routing updates may be lost or not processed in a timely manner

ITE PC v4.0


Routing Loops

Count to Infinity

This is a routing loop whereby packets bounce infinitely around a network.

ITE PC v4.0


Routing Loops

Setting a maximum

Distance Vector routing protocols set a specified metric value to indicate infinity

Once a router “counts to infinity” it marks the route as unreachable

ITE PC v4.0


Routing Loops

Preventing loops with holddown timers

-Holddown timers allow a router to not accept any changes to a route for a specified period of time.

-Point of using holddown timers

Allows routing updates to propagate through network with the most current information.

ITE PC v4.0


Routing Loops

The Split Horizon Rule is used to prevent routing loops

Split Horizon rule:

A router should not advertise a network through the interface from which the update came.

ITE PC v4.0


Routing Loops

Split horizon with poison reverse

The rule states that once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface

ITE PC v4.0


Routing Loops IP & TTL

–Purpose of the TTL field

The TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network

How the TTL field works

-TTL field contains a numeric value

The numeric value is decreased by one by every router on the route to the destination.

If numeric value reaches 0 then Packet is discarded.

ITE PC v4.0


Routing Protocols Today

Factors used to determine whether to use RIP or EIGRP include

-Network size

-Compatibility between models of routers

-Administrative knowledge

ITE PC v4.0



RIP

Features of RIP:

-Supports split horizon & split horizon with poison reverse

-Capable of load balancing

-Easy to configure

-Works in a multi vendor router environment

ITE PC v4.0



EIGRP

Features of EIGRP:

-Triggered updates

-EIGRP hello protocol used to establish neighbor adjacencies

-Supports VLSM & route summarization

-Use of topology table to maintain all routes

-Classless distance vector routing protocol

-Cisco proprietary protocol

ITE PC v4.0


Summary

Characteristics of Distance Vector routing protocols

–Periodic updates

–RIP routing updates include the entire routing table

–Neighbors are defined as routers that share a link and are configured to use the same protocol

The network discovery process for D.V. routing protocol

–Directly connected routes are placed in routing table 1st

–If a routing protocol is configured then

•Routers will exchange routing information

–Convergence is reached when all network routers have the

same network information

ITE PC v4.0


Summary D.V. routing protocols maintains routing tables by

–RIP sending out periodic updates

–RIP using 4 different timers to ensure information is accurate and convergence is achieved in a timely manner

–EIGRP sending out triggered updates

D.V. routing protocols may be prone to routing loops

– routing loops are a condition in which packets continuously traverse a network

–Mechanisms used to minimize routing loops include defining maximum hop count, holddown timers, split horizon, route poisoning and triggered updates

ITE PC v4.0


Summary

Conditions that can lead to routing loops include

–Incorrectly configured static routes

–Incorrectly configured route redistribution

–Slow convergence

–Incorrectly configured discard routes

How routing loops can impact network performance includes:

–Excess use of bandwidth

–CPU resources may be strained

–Network convergence is degraded

–Routing updates may be lost or not processed

ITE PC v4.0


Summary

Routing Information Protocol (RIP)

A distance vector protocol that has 2 versions

RIPv1 – a classful routing protocol

RIPv2 - a classless routing protocol

Enhanced Interior Gateway Routing Protocol (EIGRP)

–A distance vector routing protocols that has some features of link state routing protocols

–A Cisco proprietary routing protocol

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ITE PC v4.0

Chapter 1 143

RIP version 1


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Objectives Describe the functions, characteristics, and operation

of the RIPv1 protocol.

Configure a device for using RIPv1.

Verify proper RIPv1 operation.

Describe how RIPv1 performs automatic summarization.

Configure, verify, and troubleshoot default routes propagated in a routed network implementing RIPv1.

Use recommended techniques to solve problems related to RIPv1

ITE PC v4.0


RIPv1

RIP Characteristics

-A classful, Distance Vector (DV) routing protocol

-Metric = hop count

-Routes with a hop count > 15 are unreachable

-Updates are broadcast every 30 seconds

ITE PC v4.0


RIPv1 RIP Message Format

RIP header - divided into 3 fields

-Command field

-Version field

-Must be zero

Route Entry - composed of 3 fields

-Address family identifier

-IP address

-Metric

ITE PC v4.0


RIPv1

RIP Operation

–RIP uses 2 message types:

Request message

-This is sent out on startup by each RIP enabled interface

-Requests all RIP enabled neighbors to send routing table

Response message

-Message sent to requesting router containing routing table

ITE PC v4.0


RIPv1

IP addresses initially divided into classes

-Class A

-Class B

-Class C

RIP is a classful routing protocol

-Does not send subnet masks in routing updates

ITE PC v4.0


RIPv1

Administrative Distance

–RIP’s default administrative distance is 120

ITE PC v4.0


Basic RIPv1 Configuration A typical topology suitable for

use by RIPv1 includes:

-Three router set up

-No PCs attached to LANs

-Use of 5 different IP subnets

ITE PC v4.0


Basic RIPv1 Configuration

Router RIP Command

–To enable RIP enter:

-Router rip at the global configuration prompt

-Prompt will look like R1(config-router)#

ITE PC v4.0


Basic RIPv1 Configuration

Specifying Networks

–Use the networkcommand to:

-Enable RIP on all interfaces that belong to this network

-Advertise this network in RIP updates sent to other routers every 30 seconds

ITE PC v4.0


Verification and Troubleshooting

Show ip Route

To verify and troubleshoot routing

-Use the following

commands:

-show ip route

-show ip protocols

-debug ip rip

ITE PC v4.0



show ip protocolscommand

-Displays routing protocol configured on router

ITE PC v4.0


Verification and Troubleshooting Debug ip rip command

-Used to display RIP routing updates as they are happening

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Passive interface command

-Used to prevent a router from sending updates through an interface

-Example:

Router(config-router)#passive-interface interface-type interface-number

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Verification and Troubleshooting Passive interfaces

ITE PC v4.0


Automatic Summarization Modified Topology

The original scenario has been modified such that:

Three classful networks are used:

172.30.0.0/16

192.168.4.0/24

192.168.5.0/24

The 172.30.0.0/16 network is subnetted into three subnets:

172.30.1.0/24

172.30.2.0/24

172.30.3.0/24

The following devices are part of the 172.30.0.0/16 classful network address:

All interfaces on R1

S0/0/0 and Fa0/0 on R2

ITE PC v4.0


Automatic Summarization

Configuration Details

-To remove the RIP routing process use the following command

No router rip

-To check the configuration use the following command

Show run

ITE PC v4.0


Automatic Summarization Boundary Routers

–RIP automatically summarizes classful networks

–Boundary routers summarize RIP subnets from one major network to another.

ITE PC v4.0



Processing RIP Updates

2 rules govern RIPv1 updates:

-If a routing update and the interface it’s received on belong to the samenetwork then

The subnet mask of the interface is applied to the network in the routing update

-If a routing update and the interface it’s received on belong to a differentnetwork then

The classful subnet mask of the network is applied to the network in the routing update.

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Sending RIP Updates

–RIP uses automatic summarization to reduce the size of a routing table.

ITE PC v4.0



Advantages of automatic summarization:

-The size of routing updates is reduced

-Single routes are used to represent multiple routes which results in faster lookup in the routing table.

ITE PC v4.0


Automatic Summarization Disadvantage of Automatic Summarization:

-Does not support discontiguous networks

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Discontiguous Topologies do not converge with RIPv1

A router will only advertise major network addresses out interfaces that do not belong to the advertised route.

ITE PC v4.0


Default Route and RIPv1

Modified Topology: Scenario C

Default routes

Packets that are not defined specifically in a routing table will go to the specified interface for the default route

Example: Customer routers use default routes to connect to an ISP router.

Command used to configure a default route is

ip route 0.0.0.0 0.0.0.0 s0/0/1

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ITE PC v4.0



Propagating the Default Route in RIPv1

Default-information originate command

-This command is used to specify that the router is to originate default information, by propagating the static default route in RIP update.

ITE PC v4.0


Summary

RIP characteristics include:

Classful, distance vector routing protocol

Metric is Hop Count

Does not support VLSM or discontiguous subnets

Updates every 30 seconds

Rip messages are encapsulated in a UDP segment with source and destination ports of 520

ITE PC v4.0


Summary: Commands used by RIP

Command Command’s purpose

Rtr(config)#router rip Enables RIP routing process

Rtr(config-router)#network Associates a network with a RIP routing process

Rtr#debug ip rip used to view real time RIP routing updates

Rtr(config-router)#passive-interface fa0/0 Prevent RIP updates from going out an interface

Rtr(config-router)#default-information originate Used by RIP to propagate default routes

Rtr#show ip protocols Used to display timers used by RIP

ITE PC v4.0



ITE PC v4.0

Chapter 1 172

VLSM and CIDR


ITE PC v4.0


Objectives Compare and contrast classful and classless IP

addressing.

Review VLSM and explain the benefits of classless IP addressing.

Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses

ITE PC v4.0


Introduction Prior to 1981, IP addresses used only the first 8 bits to

specify the network portion of the address

In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes

IP address space was depleting rapidly

the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR)

–CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space.

-VLSM is simply subnetting a subnet

ITE PC v4.0


Classful and Classless IP Addressing Classful IP addressing

As of January 2007, there are over 433 million hosts on internet

Initiatives to conserve IPv4 address space include:

-VLSM & CIDR notation (1993, RFC 1519)

-Network Address Translation (1994, RFC 1631)

-Private Addressing (1996, RFC 1918)

ITE PC v4.0


Classful and Classless IP Addressing

The High Order Bits

These are the leftmost bits in a 32 bit address

ITE PC v4.0


Classful and Classless IP Addressing Classes of IP addresses are identified by the decimal number of

the 1st octet

Class A address begin with a 0 bit

Range of class A addresses = 0.0.0.0 to 127.255.255.255

Class B address begin with a 1 bit and a 0 bit

Range of class B addresses = 128.0.0.0 to 191.255.255.255

Class C addresses begin with two 1 bits & a 0 bit

Range of class C addresses = 192.0.0.0 to 223.255.255.255.

ITE PC v4.0



The IPv4 Classful Addressing Structure (RFC 790)

An IP address has 2 parts:

-The network portion

Found on the left side of an IP address

-The host portion

Found on the right side of an IP address

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ITE PC v4.0



Purpose of a subnet mask

It is used to determine the network portion of an IP address

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Classful and Classless IP Addressing Classful Routing Updates

-Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates

The reason is that the Subnet mask isdirectly related to the network address

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Classless Inter-domain Routing (CIDR – RFC 1517)

Advantage of CIDR :

-More efficient use of IPv4 address space

-Route summarization

Requires subnet mask to be included in routing update because address class is meaningless

Recall purpose of a subnet mask:

-To determine the network and host portion of an IP address

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Classless IP Addressing

CIDR & Route Summarization

-Variable Length Subnet Masking (VLSM)

-Allows a subnet to be further sub-netted according to individual needs

-Prefix Aggregation a.k.a. Route Summarization

-CIDR allows for routes to be summarized as a single route

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Classless Routing Protocol

Characteristics of classless routing protocols:

-Routing updates include the subnet mask

-Supports VLSM

Supports Route Summarization

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Routing

Protocol

Routing

updates

Include

subnet

Mask

Supports

VLSM

Ability to send

Supernet routes

Classful No No No

Classless Yes Yes Yes

Classless Routing Protocol

ITE PC v4.0


VLSM Classful routing

-only allows for one subnet mask for all networks

VLSM & classless routing

-This is the process of subnetting a subnet

-More than one subnet mask can be used

-More efficient use of IP addresses as compared to classful IP addressing

ITE PC v4.0


VLSM

VLSM – the process of sub-netting a subnet to fit your needs

-Example:

Subnet 10.1.0.0/16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask.

-Mask allows for 254 host addresses per subnet

-Subnets range from: 10.1.0.0 / 24 to 10.1.255.0 / 24

ITE PC v4.0


Classless Inter-Domain Routing (CIDR) Route summarization done by CIDR

-Routes are summarized with masks that are less than that of the default classful mask

-Example:

172.16.0.0 / 13 is the summarized route for the 172.16.0.0 / 16 to 172.23.0.0 / 16 classful networks

ITE PC v4.0


Classless Inter-Domain Routing (CIDR)

Steps to calculate a route summary

-List networks in binary format

-Count number of left most matching bits to determine summary route’s mask

-Copy the matching bits and add zero bits to determine the summarized network address

ITE PC v4.0


Summary Classful IP addressing

IPv4 addresses have 2 parts:

-Network portion found on left side of an IP address

-Host portion found on right side of an IP address

Class A, B, & C addresses were designed to provide IP addresses for different sized organizations

The class of an IP address is determined by the decimal value found in the 1st octet

IP addresses are running out so the use of Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are used to try and conserve address space

ITE PC v4.0


Summary

Classful Routing Updates

–Subnet masks are not sent in routing updates

Classless IP addressing

–Benefit of classless IP addressing

Can create additional network addresses using a subnet mask that fits your needs

–Uses Classless Interdomain Routing (CIDR)

ITE PC v4.0


Summary

CIDR

Uses IP addresses more efficiently through use of VLSM

-VLSM is the process of subnetting a subnet

Allows for route summarization

-Route summarization is representing multiple contiguous routes with a single route

ITE PC v4.0


Summary

Classless Routing Updates

Subnet masks are included in updates

ITE PC v4.0



ITE PC v4.0

Chapter 1 195

RIPv2


ITE PC v4.0


Objectives Encounter and describe the limitations of RIPv1’s

limitations.

Apply the basic Routing Information Protocol Version 2 (RIPv2) configuration commands and evaluate RIPv2 classless routing updates.

Analyze router output to see RIPv2 support for VLSM and CIDR

Identify RIPv2 verification commands and common RIPv2 issues.

Configure, verify, and troubleshoot RIPv2 in “hands-on” labs

ITE PC v4.0


Introduction

Chapter focus

-Difference between RIPv1 & RIPv2

RIPv1

-A classful distance vector routing protocol

-Does not support discontiguous subnets

-Does not support VLSM

-Does not send subnet mask in routing update

-Routing updates are broadcast

RIPv2

-A classless distance vector routing protocol that is an enhancement of RIPv1’s features.

-Next hop address is included in updates

-Routing updates are multicast

-The use of authentication is an option

ITE PC v4.0


Introduction

Similarities between RIPv1 & RIPv2

-Use of timers to prevent routing loops

-Use of split horizon or split horizon with poison reverse

-Use of triggered updates

-Maximum hop count of 15

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RIPv1 Limitations

Lab Topology

Scenario:

3 router set up

Topology is discontiguous

There exists a static summary route

Static route information can be injected into routing table updates using redistribution.

Routers 1 & 3 contain VLSM networks

ITE PC v4.0


RIPv1 Limitations Scenario Continued

VLSM

-Recall this is sub netting the subnet

Private IP addresses are on LAN links

Public IP addresses are used on WAN links

Loopback interfaces

-These are virtual interfaces that can be pinged and added to routing table

ITE PC v4.0


RIPv1 Limitations Null Interfaces

This is a virtual interface that does not need to be created or configured

-Traffic sent to a null interface is discarded

-Null interfaces do not send or receive traffic

Static routes and null interfaces

null interfaces will serve as the exit interface for static route

-Example of configuring a static supernet route with a

null interface

-R2(config)#ip route 192.168.0.0 255.255.0.0 Null0

ITE PC v4.0


RIPv1 Limitations Route redistribution

-Redistribution command is way to disseminate a static route from one router to another via a routing protocol

-Example

R2(config-router)#redistribute static

ITE PC v4.0


RIPv1 Limitations

Verifying and Testing Connectivity

Use the following commands:

show ip interfaces brief

ping

traceroute

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RIPv1 Limitations

RIPv1 – a classful routing protocol

-Subnet mask are not sent in updates

-Summarizes networks at major network boundaries

-if network is discontiguous and RIPv1 configured convergence will not be reached

ITE PC v4.0


RIPv1 Limitations

Examining the routing tables

-To examine the contents of routing updates use the

debug ip rip command

-If RIPv1 is configured then

Subnet masks will not be included with the network address

ITE PC v4.0


RIPv1 Limitations RIPv1 does not support

VLSM

Reason: RIPv1 does not send subnet mask in routing updates

RIPv1 does summarize routes to the Classful boundary

Or uses the Subnet mask of the outgoing interface to determine which subnets to advertise

ITE PC v4.0


RIPv1 Limitations

No CIDR Support

In the diagram R2 will not include the static route in its update

Reason: Classful routing protocols do not support CIDR routes that are summarized with a smaller mask than the classful subnet mask

ITE PC v4.0


Configuring RIPv2

Comparing RIPv1 & RIPv2 Message Formats

-RIPv2 Message format is similar to RIPv1 but has 2 extensions

1st extension is the subnet mask field

2nd extension is the addition of next hop address

ITE PC v4.0


Configuring RIPv2

Enabling and Verifying RIPv2

Configuring RIP on a Cisco router

By default it is running RIPv1

ITE PC v4.0


Configuring RIPv2

Configuring RIPv2 on a Cisco router

-Requires using the version 2 command

-RIPv2 ignores RIPv1 updates

To verify RIPv2 is configured use the

show ip protocolscommand

ITE PC v4.0


Configuring RIPv2

Auto-Summary & RIPv2

RIPv2 will automatically summarize routes at major network boundaries and can also summarize routes with a subnet mask that is smaller than the classful subnet mask

ITE PC v4.0


Disabling Auto-Summary in RIPv2

To disable automatic summarization issue the no auto-summarycommand

Configuring RIPv2

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Configuring RIPv2

Verifying RIPv2 Updates

When using RIPv2 with automatic summarization turned off

Each subnet and mask has its own specific entry, along with the exit interface and next-hop address to reach that subnet.

To verify information being sent by RIPv2 use the

debug ip rip command

ITE PC v4.0


VLSM & CIDR

RIPv2 and VLSM

Networks using a VLSM IP addressing scheme

Use classless routing protocols (i.e. RIPv2) to disseminate network addresses and their subnet masks

ITE PC v4.0


VLSM & CIDR

CIDR uses Supernetting

Supernetting is a bunch of contiguous classful networks that is addressed as a single network.

ITE PC v4.0


VLSM & CIDR

To verify that supernets are being sent and received use the following commands

-Show ip route

-Debug ip rip

ITE PC v4.0


Verifying & Troubleshooting RIPv2

Basic Troubleshooting steps

-Check the status of all links

-Check cabling

-Check IP address & subnet mask configuration

-Remove any unneeded configuration commands

Commands used to verify proper operation of RIPv2

–Show ip interfaces brief

–Show ip protocols

–Debug ip rip

–Show ip route

ITE PC v4.0



Common RIPv2 Issues

When trouble shooting RIPv2 examine the following issues:

Version

Check to make sure you are using version 2

Network statements

Network statements may be incorrectly typed or missing

Automatic summarization

If summarized routes are not needed then disable automatic summarization

ITE PC v4.0



Reasons why it’s good to authenticate routing information

-Prevent the possibility of accepting invalid routing updates

-Contents of routing updates are encrypted

Types of routing protocols that can use authentication

-RIPv2

-EIGRP

-OSPF

-IS-IS

-BGP

ITE PC v4.0


Summary

Routing

Protocol

Distance

Vector

Classless

Routing

Protocol

Uses

Hold-

Down

Timers

Use of

Split

Horizon

or

Split

Horizon

w/

Poison

Reverse

Max

Hop

count

= 15

Auto

Summary

Support

CIDR

Supports

VLSM

Uses

Authen-

tication

RIPv1 Yes No Yes Yes Yes Yes No No No

RIPv2 Yes Yes Yes Yes Yes Yes Yes Yes Yes

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ITE PC v4.0

Chapter 1 222

The Routing Table: A Closer Look


ITE PC v4.0


Objectives Describe the various route types found in the routing

table structure

Describe the routing table lookup process.

Describe routing behavior in routed networks.

ITE PC v4.0


Introduction

Chapter Focus

-Structure of the routing table

-Lookup process of the routing table

-Classless and classful routing behaviors

ITE PC v4.0


Routing Table Structure

Lab Topology

3 router setup

-R1 and R2 share a common 172.16.0.0/16 network with 172.16.0.0/24 subnets.

-R2 and R3 are connected by the 192.168.1.0/24 network.

-R3 also has a 172.16.4.0/24 subnet, which is disconnected, or discontiguous, from the 172.16.0.0 network that R1 and R2 share.

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Routing table entries come from the following sources


-Static routes

-Dynamic routing protocols

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Level 1 Routes

As soon as the no shutdown command is issued the route is added to routing table

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Cisco IP routing table is a hierarchical structure

-The reason for this is to speed up lookup process

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Level 1 Routes

-Have a subnet mask equal to or less than the classful mask of the network address.

Level 1 route can function as

-Default route

-Supernet route

-Network route

ITE PC v4.0


Routing Table Structure Level 1 Routes

-Ultimate RouteIncludes either:

-A next-hop address

OR

-An exit interface

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Parent and Child Routes

-A parent route is a level 1 route

-A parent route does not contain any next-hop IP address or exit interface information


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Automatic creation of parent routes

-Occurs any time a subnet is added to the routing table

Child routes

-Child routes are level 2 routes

-Child routes are asubnet of a classful network address

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Level 2 child routes contain route source & the network address of the route

Level 2 child routes are also considered ultimate routes

Reason: they contain the next hop address &/or exit interface

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Both child routes have the same subnet mask

-This means the parent route maintains the /24 mask

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Diagram illustrates 2 child networks belonging to the parent route 172.16.0.0 / 24

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In classless networks, child routes do not have to share the same subnet mask

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Network

Type

Parent route’s

Classful mask is

Displayed

Term

variably

subnetted

is seen in parent

route in routing

table

Includes the

# of different

masks of

child routes

Subnet mask

included

with each

child route

entry

Class-

ful

No No No No

Class-

less

Yes Yes Yes Yes

Parent & Child Routes: Classless Networks

ITE PC v4.0



Parent & Child Routes: Classless Networks

ITE PC v4.0


Routing Table Lookup Process The Route Lookup Process

Examine level 1 routes

-If best match a level 1 ultimate route and is not a parent route this route is used to forward packet

Router examines level 2 (child) routes

-If there is a match with level 2 child route then

that subnet is used to forward packet

-If no match then

determine routing behavior type

Router determines classful or classless routing behavior

-If classful then

packet is dropped

-If classless then router searches level one supernet and default routes

-If there exists a level 1 supernet or default route match then Packet is forwarded. If not packet is dropped

ITE PC v4.0


Routing Table Lookup Process

Longest Match: Level 1 Network Routes

–Best match is also known as the longest match

–The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.

ITE PC v4.0



Finding the subnet mask used to determine the longest match

Scenario:

–PC1 pings 192.168.1.2

–Router examines level 1 route for best match

–There exist a match between192.168.1.2 & 192.168.1.0 / 24

–Router forwards packets out s0/0/0

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The process of matching

-1st there must be a match made between the parent route & destination IP

-If a match is made then an attempt at finding a match between the destination IP and the child route

is made.

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Finding a match between packet’s destination IP address and the next route in the routing table

-The figure shows a match between the destination IP of 192.168.1.0 and the level one IP of 192.168.1.0 / 24 then packet forwarded out s0/0/0

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Level 1 Parent & Level 2 Child Routes

Before level 2 child routes are examined

-There must be a match between classful level one parent route and destination IP address.

ITE PC v4.0


Routing Table Lookup Process After the match with parent route has been made Level 2 child

routes will be examined for a match

-Route lookup process searches for child routes with a match with destination IP

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How a router finds a match with one of the level 2 child routes

-First router examines parent routes for a match

-If a match exists then:

Child routes are examined

Child route chosen is the one with the longest match

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Example: Route Lookup Process with VLSM

-The use of VLSM does not change the lookup process

-If there is a match between destination IP address and the level 1 parent route then

-Level 2 child routes will be searched

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Routing Behavior Classful & classless routing protocols

Influence how routing table is populated

Classful & classless routing behaviors

Determines how routing table is searched after it is filled

ITE PC v4.0


Routing Behavior Classful Routing

Behavior: no ip classless

What happens if there is not a match with any level 2 child routes of the parent?

-Router must determine if the routing behavior is classless or classful

-If router is utilizing classful routing behavior then

-Lookup process is terminated and packet is dropped

ITE PC v4.0


Routing Behavior

Classful Routing Behavior – Search Process

An example of when classful routing behavior is in effect and why the router drops the Packet

-The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits. This means packet is dropped

ITE PC v4.0


Routing Behavior

Classful Routing Behavior – Search Process

The reason why the router will not search beyond the child routes

Originally networks were all classful

This meant an organization could subnet a major network address and “enlighten” all the organization’s routers about the subnetting

Therefore, if the subnet was not in the routing table, the subnet did not exist and packet was dropped

ITE PC v4.0


Routing Behavior

ip Classless

Beginning with IOS 11.3, ip classless was configured by default

Classless routing behavior works for

-Discontiguous networks

And

-CIDR supernets

ITE PC v4.0


Routing Behavior

Classless Routing Behavior: ip classless

Route lookup process when ip classless is in use

-If classless routing behavior in effect then

Search level 1 routes

Supernet routes Checked first

-If a match exists then forward packet

Default routes Checked second

If there is no match or no default route then the

Packet is dropped

ITE PC v4.0


Routing Behavior

Classless Routing Behavior – Search Process

Router begins search process by finding a match between destination IP and parent route

After finding the above mentioned match, then there is a search of the child route

ITE PC v4.0


Routing Behavior

Classless Routing Behavior – Search Process

If no match is found in child routes of previous slide then

Router continues to search the routing table for a match that may have fewer bits in the match

ITE PC v4.0


Routing Behavior

Classful vs. Classless Routing Behavior

-It is recommended to use classless routing behavior

Reason: so supernet and default routes can be used whenever needed

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Summary

Content/structure of a routing table

Routing table entries


-Static route

-Dynamic routing protocols

Routing tables are hierarchical

-Level 1 route

Have a subnet mask that is less than or equal to classful subnet mask for the network address

-Level 2 route

These are subnets of a network address

ITE PC v4.0


SummaryRouting table lookup process

Begins with examining level 1 routes for best match with packet’s destination IP

If the best match = an ultimate route then

-Packet is forwarded -Else-

-Parent route is examined

If parent route & destination IP match then Level 2 (child) routes are examined

Level 2 route examination

If a match between destination IP and child route found then Packet forwarded -Else

If Router is using classful routing behavior then Packet is dropped -Else

If router is using classless routing behavior then

Router searches Level 1 supernet & default routes for a match

If a match is found then Packet if forwarded -Else

Packet is dropped

ITE PC v4.0


Summary

Routing behaviors

-This refers to how a routing table is searched

Classful routing behavior

-Indicated by the use of the no ip classless command

-Router will not look beyond child routes for a lesser match

Classless routing behavior

-Indicated by the use of the ip classless command

-Router will look beyond child routes for a lesser match

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ITE PC v4.0

Chapter 1 261

EIGRP


ITE PC v4.0


Objectives Describe the background and history of Enhanced

Interior Gateway Routing Protocol (EIGRP).

Examine the basic EIGRP configuration commands and identify their purposes.

Calculate the composite metric used by EIGRP.

Describe the concepts and operation of DUAL.

Describe the uses of additional configuration commands in EIGRP.

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Introduction

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EIGRP

Roots of EIGRP: IGRP

-Developed in 1985 to overcome RIPv1’s limited hop count

-Distance vector routing protocol

-Metrics used by IGRP

bandwidth (used by default)

Delay (used by default)

reliability

load

-Discontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S

ITE PC v4.0


EIGRP

EIGRP Message Format

EIGRP Header

Data link frame header - contains source and destination MAC address

IP packet header - contains source & destination IP address

EIGRP packet header - contains AS number

Type/Length/Field - data portion of EIGRP message

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EIGRP

EIGRP packet headercontains

–Opcode field

–Autonomous System number

EIGRP Parameters contains

–Weights

–Hold time

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EIGRP

TLV: IP internal contains

–Metric field

–Subnet mask field

–Destination field

TLV: IP external contains

–Fields used when external

routes are imported into

EIGRP routing process

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EIGRPProtocol Dependent

Modules (PDM)

EIGRP uses PDM to route several different protocols i.e. IP, IPX & AppleTalk

PDMs are responsible for the specific routing task for each network layer protocol

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EIGRPReliable Transport Protocol (RTP)

Purpose of RTP

–Used by EIGRP to transmit and receive EIGRP packets

Characteristics of RTP

–Involves both reliable & unreliable delivery of EIGRP packet

Reliable delivery requires acknowledgment from destination

Unreliable delivery does not require an acknowledgement from destination

–Packets can be sent

Unicast

Multicast

–Using address 224.0.0.10

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EIGRP

EIGRP’s 5 Packet Types

Hello packets

–Used to discover & form adjacencies with neighbors

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EIGRP

Update packets

–Used to propagate routing information

Acknowledgement packets

–Used to acknowledge receipt of update, query & reply packets

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EIGRP

Query & Reply packets

Used by DUAL for searching for networks

Query packets

-Can use

Unicast

Multicast

Reply packet

-Use only

unicast

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EIGRP Purpose of Hello Protocol

–To discover & establish adjacencies with neighbor routers

Characteristics of hello protocol

–Time interval for sending hello packet

Most networks it is every 5 seconds

Multipoint non broadcast multi-access networks

–Unicast every 60 seconds

-Holdtime

This is the maximum time router should wait before declaring a neighbor down

Default holdtime

–3 times hello interval

ITE PC v4.0


EIGRPEIGRP Bounded Updates

EIGRP only sends update when there is a change inroute status

Partial update

–A partial update includes only the route information that has changed – the whole routing table is NOT sent

Bounded update

–When a route changes, only those devices that are impacted will be notified of the change

EIGRP’s use of partial bounded updates minimizes use of bandwidth

ITE PC v4.0


EIGRP

Diffusing Update Algorithm (DUAL)

–Purpose

•EIGRP’s primary method for preventing routing loops

–Advantage of using DUAL

•Provides for fast convergence time by keeping a list of loop-free backup routes

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EIGRP

Administrative Distance (AD)

–Defined as the trustworthiness of the source route

EIGRP default administrative distances

–Summary routes = 5

–Internal routes = 90

–Imported routes = 170

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EIGRP

Authentication

EIGRP can

– Encrypt routing information

– Authenticate routing information

ITE PC v4.0


EIGRP

Network Topology

Topology used is the same as previous chapters with the addition of an ISP router

ITE PC v4.0


EIGRP

EIGRP will automatically summarize routes at classful boundaries

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Basic EIGRP Configuration

Autonomous System (AS) & Process IDs

–This is a collection of networks under the control of a single authority (reference RFC 1930)

–AS Numbers are assigned by IANA

–Entities needing AS numbers

ISP

Internet Backbone prodiers

Institutions connecting to other institutions using AS numbers

ITE PC v4.0



EIGRP autonomous system number actually functions as a process ID

Process ID represents an instance of the routing protocol running on a router

Example

Router(config)#router

eigrp autonomous-system

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The router eigrp command

The global command that enables eigrp is

router eigrp autonomous-system

-All routers in the EIGRP routing domain must use the same process ID number (autonomous-system

number)

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The Network Command

Functions of the network command

–Enables interfaces to transmit & receive EIGRP updates

–Includes network or subnet in EIGRP updates

Example

–Router(config-router)#network network-address

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The network Command with a Wildcard Mask

-This option is used when you want to configure EIGRP to advertise specific subnets

-Example

Router(config-router)#network network-address [wildcard-mask]

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Verifying EIGRP

EIGRP routers must establish adjacencies with their neighbors before any updates can be sent or received

Command used to view neighbor table and verify that EIGRP has established adjacencies with neighbors is

show ip eigrp neighbors

ITE PC v4.0


EIGRP

The show ip protocolscommand is also used to verify that EIGRP is enabled

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Examining the Routing Table

The show ip routecommand is also used to verify EIGRP

EIGRP routes are denoted in a routing table by the letter “D”

By default , EIGRP automatically summarizes routes at major network boundary

ITE PC v4.0


Basic EIGRP Configuration Introducing the Null0 Summary Route

–Null0 is not a physical interface

–In the routing table summary routes are sourced from Null0

Reason: routes are used for advertisement purposes

–EIGRP will automatically include a null0 summary route as child route when 2 conditions are met

At least one subnet is learned via EIGRP

Automatic summarization is enabled

ITE PC v4.0



R3’s routing table shows that the 172.16.0.0/16 network is automatically summarized by R1 & R3

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EIGRP Metric CalculationEIGRP Composite Metric & the K Values

EIGRP uses the following values in its composite metric

-Bandwidth, delay, reliability, and load

The composite metric used by EIGRP– formula used has values K1 K5

K1 & K3 = 1

all other K values = 0

ITE PC v4.0


EIGRP Metric Calculation

Use the sh ip protocols command to verify the K values

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EIGRP Metrics

Use the show interfaces command to view metrics

EIGRP Metrics

Bandwidth – EIGRP uses a static bandwidth to calculate metric

Most serial interfaces use a default bandwidth value of 1.544Mbos (T1)

ITE PC v4.0


EIGRP Metric CalculationEIGRP Metrics

Delay is the defined as the measure of time it takes for a packet to traverse a route

-it is a static value based on link type to which interface is connected

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Reliability (not a default EIGRP metric)

-A measure of the likelihood that a link will fail

-Measure dynamically & expressed as a fraction of 255

the higher the fraction the better the reliability

Load (not a default EIGRP metric)

– A number that reflects how much traffic is using a link

– Number is determined dynamically and is expressed as a fraction of 255

The lower the fraction the less the load on the link

ITE PC v4.0



Using the Bandwidth Command

Modifying the interface bandwidth

-Use the bandwidth command

-Example

Router(config-if)#bandwidth kilobits

Verifying bandwidth

–Use the show interface command

Note – bandwidth command

does not change the

link’s physical

bandwidth

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The EIGRP metric can be determined by examining the

bandwidth delay

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EIGRP uses the lowest bandwidth (BW)in its metric calculation

Calculated BW = reference BW / lowest BW(kbps)

Delay – EIGRP uses the cumulative sum of all outgoing interfaces

Calculated Delay = the sum of outgoing interface delays

EIGRP Metric = calculated BW + calculated delay

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ITE PC v4.0


DUAL Concepts

The Diffusing Update Algorithm (DUAL) is used to prevent looping

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DUAL Concepts

Successor

The best least cost route to a destination found in the routing table

Feasible distance

The lowest calculated metricalong a path to a destination network

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DUAL Concepts

Feasible Successors, Feasibility Condition & Reported Distance

Feasible Successor

-This is a loop free backup route to same

destination as successor route

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DUAL Concepts

Reported distance (RD)

-The metric that a router reports to a neighbor about its own cost to that network

Feasible Successors, Feasibility Condition & Reported Distance

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DUAL Concepts

Feasibility Condition (FC)

-Met when a neighbor’s RD is less thanthe local router’s FD to the same destination network

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DUAL Concepts

Topology Table: Successor & Feasible Successor

EIGRP Topology table

–Viewed using the show ip eigrp topology command

Contents of table include:

– all successor routes

– all feasible successor routes

ITE PC v4.0


DUAL Concepts

EIGRP Topology Table dissected

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DUAL Concepts

Topology Table: No Feasible Successor

A feasible successor may not be present because the feasibility condition may not be met

-In other words, the reported distance of the neighbor is greater than or equal to the current feasible distance

ITE PC v4.0


DUAL Concepts

Finite Sate Machine (FSM)

–An abstract machine that defines a set of possible states something can go through, what event causes those states and what events result form those states

–FSMs are used to describe how a device, computer program, or routing algorithm will react to a set of input events

ITE PC v4.0


DUAL Concepts

DUAL FSM

–Selects a best loop-free path to a destination

–Selects alternate routes by using information in EIGRP tables

ITE PC v4.0


DUAL Concepts

Finite State Machines (FSM)

To examine output from EIGRP’s finite state machine us the debug eigrp fsm command

ITE PC v4.0


More EIGRP Configurations

The Null0 Summary Route

By default, EIGRP uses the Null0 interface to discard any packets that match the parent route but do not match any of the child routes

EIGRP automatically includes a null0 summary route as a child route whenever both of the following conditions exist

–One or subnets exists that was learned via EIGRP

–Automatic summarization is enabled

ITE PC v4.0



The Null0 Summary Route

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Disabling Automatic Summarization

The auto-summary command permits EIGRP to automatically summarize at major network boundaries

The no auto-summary command is used to disable automatic summarization

–This causes all EIGRP neighbors to send updates that will not be automatically summarized

this will cause changes to appear in both

-routing tables

-topology tables

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Manual Summarization

Manual summarization can include supernets

Reason: EIGRP is a classless routing protocol & include subnet mask in update

Command used to configure manual summarization

–Router(config-if)#ip summary-address eigrp as-number network-address subnet-mask

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Configuring a summary route in EIGRP

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EIGRP Default Routes

“quad zero” static default route

-Can be used with any currently supported routing protocol

-Is usually configured on a router that is connected a network outside the EIGRP domain

EIGRP & the “Quad zero” static default route

–Requires the use of the redistribute static command to disseminate default route in EIGRP updates

ITE PC v4.0



Fine-Tuning EIGRP

EIGRP bandwidth utilization

-By default, EIGRP uses only up to 50% of interface bandwidth for EIGRP information

-The command to change the percentage of bandwidth used by EIGRP is

Router(config-if)#ip bandwidth-percent eigrp as-number percent

ITE PC v4.0


More EIGRP Configurations Configuring Hello Intervals and Hold Times

-Hello intervals and hold times are configurable on a per-interface basis

-The command to configure hello interval is

Router(config-if)#ip hello-interval eigrp as-number seconds

Changing the hello interval also requires changing the hold time to a value greater than or equal to the hello interval

-The command to configure hold time value is

Router(config-if)#ip hold-time eigrp as-number seconds

ITE PC v4.0


Summary Background & History

–EIGRP is a derivative of IGRP

EIGRP is a Cisco proprietary distance vector routing protocol released in 1994

EIGRP terms and characteristics

–EIGPR uses RTP to transmit & receive EIGRP packets

–EIGRP has 5 packet type:

Hello packets

Update packets

Acknowledgement packets

Query packets

Reply packets

–Supports VLSM & CIDR

ITE PC v4.0


Summary

EIGRP terms and characteristics

–EIGRP uses a hello protocol

Purpose of hello protocol is to discover & establish adjacencies

–EIGRP routing updates

Aperiodic

Partial and bounded

Fast convergence

ITE PC v4.0


Summary

EIGRP commands

–The following commands are used for EIGRP configuration

RtrA(config)#router eigrp [autonomous-system #]

RtrA(config-router)#network network-number

–The following commands can be used to verify EIGRP

Show ip protocols

Show ip eigrp neighbors

Show ip route

ITE PC v4.0


Summary

EIGRP metrics include

–Bandwidth (default)

–Delay (default)

–Reliability

–Load

ITE PC v4.0


SummaryDUAL

–Purpose of DUAL

To prevent routing loops

–Successor

Primary route to a destination

–Feasible successor

Backup route to a destination

–Feasible distance

Lowest calculated metric to a destination

–Reported distance

The distance towards a destination as advertised by an upstream neighbor

ITE PC v4.0


Summary Choosing the best route

–After router has received all updates from directly connected neighbors, it can calculate its DUAL

1st metric is calculated for each route

2nd route with lowest metric is designated successor & is placed in routing table

3rd feasible successor is found

–Criteria for feasible successor: it must have lower reported distance to the destination than the installed route’s feasible distance

–Feasible routes are maintained in topology table

ITE PC v4.0


Summary

Automatic summarization

–On by default

–Summarizes routes on classful boundary

–Summarization can be disabled using the following command

RtrA(config-if)#no auto-summary

ITE PC v4.0



ITE PC v4.0

Chapter 1 326

Link-State Routing Protocols


ITE PC v4.0


Objectives Describe the basic features & concepts of link-state

routing protocols.

List the benefits and requirements of link-state routing protocols.

ITE PC v4.0


Introduction

ITE PC v4.0


Link-State Routing

Link state routing protocols

-Also known as shortest path first algorithms

-These protocols built around Dijkstra’s SPF

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Link-State Routing

Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm

ITE PC v4.0


Link-State Routing

The shortest path to a destination is not necessarily the path with the least number of hops

ITE PC v4.0


Link-State RoutingLink-State Routing Process

How routers using Link State Routing Protocols reach convergence

-Each routers learns about its own directly connected networks

-Link state routers exchange hello packet to “meet” other directly

connected link state routers.

-Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth.

-After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information.

-Once all the routers have received all the LSPs, the routers then construct a topological map of the network which is used to determine the best routes to a destination

ITE PC v4.0


Link-State Routing

Directly Connected Networks

Link

This is an interface on a router

Link state

This is the information about the state of the links

ITE PC v4.0


Link-State Routing

Sending Hello Packets to Neighbors

Link state routing protocols use a hello protocol

Purpose of a hello protocol:

-To discover neighbors (that use the same link state routing protocol) on its link

ITE PC v4.0


Link-State RoutingSending Hello Packets to

Neighbors

Connected interfaces that are using the same link state routing protocols will exchange hello packets.

Once routers learn it has neighbors they form an adjacency

-2 adjacent neighbors will exchange hello packets

-These packets will serve as a keep alive function

ITE PC v4.0


Link-State Routing

Building the Link State Packet

Each router builds its own Link State Packet (LSP)

Contents of LSP:

-State of each directly connected link

-Includes information about neighbors such as neighbor ID, link type, & bandwidth.

ITE PC v4.0


Link-State Routing

Flooding LSPs to Neighbors

Once LSP are created they are forwarded out to neighbors.

-After receiving the LSP the neighbor continues to forward it throughout routing area.

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Link-State Routing

LSPs are sent out under the following conditions

-Initial router start up or routing process

-When there is a change in topology

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Link-State Routing

Constructing a link state data base

Routers use a database to construct a topology map of the network

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Link-State Routing

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Link-State RoutingShortest Path First (SPF) Tree

Building a portion of the SPF tree

Process begins by examining R2’s LSP information

-R1 ignores 1st LSP

Reason: R1 already knows it’s connected to R2

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Link-State Routing


-R1 uses 2nd LSP

Reason: R1 can create a link from R2 to R5. This information is added to R1’s SPF tree

ITE PC v4.0


Link-State Routing


-R1 uses 3rd LSP

Reason: R1 learns that R2 is connected to 10.5.0.0/16.

This link is added to R1’s SPF tree.

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Link-State Routing

Determining the shortest path

The shortest path to a destination determined by adding the costs & finding the lowest cost

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Link-State Routing

Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table.

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Routing

protocol

Builds

Topological

map

Router can

independently

determine the

shortest path to

every network.

Convergence

A periodic/

event driven

routing updates

Use

of

LSP

Distance

vector

No No Slow Generally No No

Link State Yes Yes Fast Generally Yes Yes

Advantages of a Link-State Routing Protocol

ITE PC v4.0



Requirements for using a link state routing protocol

Memory requirements

Typically link state routing protocols use more memory

Processing Requirements

More CPU processing is required of link state routing protocols

Bandwidth Requirements

Initial startup of link state routing protocols can consume lots of bandwidth

ITE PC v4.0



2 link state routing protocols used for routing IP

-Open Shortest Path First (OSPF)

-Intermediate System-Intermediate System (IS-IS)

ITE PC v4.0


Summary

Link State Routing protocols are also known as Shortest Path First protocols

Summarizing the link state process

-Routers 1ST learn of directly connected networks

-Routers then say “hello” to neighbors

-Routers then build link state packets

-Routers then flood LSPs to all neighbors

-Routers use LSP database to build a network topology map & calculate the best path to each destination

ITE PC v4.0


Summary

Link

An interface on the router

Link State

Information about an interface such as

-IP address

-Subnet mask

-Type of network

-Cost associated with link

-Neighboring routers on the link

ITE PC v4.0


Summary

Link State Packets

After initial flooding, additional LSP are sent out when a change in topology occurs

Examples of link state routing protocols

-Open shortest path first

-IS-IS

ITE PC v4.0



ITE PC v4.0

Chapter 1 353

OSPF


ITE PC v4.0


Objectives Describe the background and basic features of OSPF

Identify and apply the basic OSPF configuration commands

Describe, modify and calculate the metric used by OSPF

Describe the Designated Router/Backup Designated Router (DR/BDR) election process in multiaccess networks

Describe the uses of additional configuration commands in OSPF

ITE PC v4.0


Introduction

ITE PC v4.0


Introduction to OSPF

Background of OSPF

Began in 1987

1989 OSPFv1 released in RFC 1131

This version was experimental & never deployed

1991 OSPFv2 released in RFC 1247

1998 OSPFv2 updated in RFC 2328

1999 OSPFv3 published in RFC 2740

ITE PC v4.0


Introduction to OSPFOSPF Message Encapsulation

OSPF packet type

There exist 5 types

OSPF packet header

Contains - Router ID and area ID and Type code for OSPF packet type

IP packet header

Contains - Source IP address, Destination IP address, & Protocol field set to 89

ITE PC v4.0


Introduction to OSPFOSPF Message Encapsulation

Data link frame header

Contains - Source MAC address and Destination MAC address

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Introduction to OSPFOSPF Packet Types

ITE PC v4.0


Introduction to OSPFHello Protocol

OSPF Hello Packet

–Purpose of Hello Packet

Discover OSPF neighbors & establish adjacencies

Advertise guidelines on which routers must agree to become neighbors

Used by multi-access networks to elect a designated router and a backup designated router

ITE PC v4.0



Hello Packets continued

Contents of a Hello Packet

router ID of transmitting router

OSPF Hello Intervals

–Usually multicast (224.0.0.5)

–Sent every 30 seconds for NBMA segments

OSPF Dead Intervals

–This is the time that must transpire

before the neighbor is considered

down

–Default time is 4 times

the hello interval

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Hello protocol packets contain information that is used in electing

-Designated Router (DR)

DR is responsible for updating all other OSPF routers

-Backup Designated Router (BDR)

This router takes over DR’s responsibilities if DR fails

ITE PC v4.0


Introduction to OSPFOSPF Link-state Updates

Purpose of a Link State Update (LSU)

Used to deliver link state advertisements

Purpose of a Link State Advertisement (LSA)

Contains information about neighbors & path costs

ITE PC v4.0


Introduction to OSPFOSPF Algorithm

OSPF routers build & maintain link-state database containing LSA received from other routers

–Information found in database is utilized upon execution of Dijkstra SPF algorithm

–SPF algorithm used to create SPF tree

–SPF tree used to populate routing table

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Administrative Distance

Default Administrative Distance for OSPF is 110

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Introduction to OSPF OSPF Authentication

–Purpose is to encrypt & authenticate routing information

–This is an interface specific configuration

–Routers will only accept routing information from other routers that have been configured with the same password or authentication information

ITE PC v4.0


Basic OSPF Configuration

Lab Topology

Topology used for this chapter

Discontiguous IP addressing scheme

Since OSPF is a classless routing protocol the subnet mask is configured in

ITE PC v4.0



The router ospf command

To enable OSPF on a router use the following command

R1(config)#router ospf process-id

Process id

A locally significant number between 1 and 65535

-this means it does not have to match other OSPF routers

ITE PC v4.0


Basic OSPF Configuration OSPF network command

-Requires entering: network address

wildcard mask - the inverse of the subnet mask

area-id - area-id refers to the OSPF area. OSPF area is a group of routers that share link state information

-Example: Router(config-router)#network network-address wildcard-ask area area-id

ITE PC v4.0



Router ID

–This is an IP address used to identify a router

–3 criteria for deriving the router ID

Use IP address configured with OSPF router-id command

-Takes precedence over loopback and physical interface addresses

If router-id command not used then router chooses highest IP address of any loopback interfaces

If no loopback interfaces are configured then the highest IP address on any active interface is used

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OSPF Router ID

Commands used to verify current router ID

–Show ip protocols

–Show ip ospf

–Show ip ospf interface

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OSPF Router ID

Router ID & Loopback addresses

-Highest loopback address will be used as router ID if router-id command isn’t used

-Advantage of using loopback address

the loopback interface cannot fail OSPF stability

The OSPF router-id command

–Introduced in IOS 12.0

–Command syntax

Router(config)#router ospfprocess-id

Router(config-router)#router-idip-address

Modifying the Router ID

–Use the command Router#clear ip ospf process

ITE PC v4.0


Basic OSPF ConfigurationVerifying OSPF

Use the show ip ospf command to verify & trouble shoot OSPF networks

Command will display the following:

Neighbor adjacency

-No adjacency indicated by -

Neighboring router’s Router ID is not displayed

A state of full is not displayed

-Consequence of no adjacency-

No link state information exchanged

Inaccurate SPF trees & routing tables

ITE PC v4.0



Command Description

Show ip protocols

Displays OSPF process ID, router ID, networks router is advertising & administrative distance

Show ip ospf

Displays OSPF process ID, router ID, OSPF area information & the last time SPF algorithm calculated

Show ip ospf interfaceDisplays hello interval and dead interval

Verifying OSPF - Additional Commands

ITE PC v4.0



Examining the routing table

Use the show ip route command to display the routing table

-An “O’ at the beginning of a route indicates that the router source is OSPF

-Note OSPF does not automatically summarize at major network boundaries

ITE PC v4.0


OSPF Metric

OSPF uses cost as the metric for determining the best route

-The best route will have the lowest cost

-Cost is based on bandwidth of an interface

Cost is calculated using the formula

108 / bandwidth

-Reference bandwidth

defaults to 100Mbps

can be modified using

auto-cost reference-bandwidth command

ITE PC v4.0


OSPF Metric COST of an OSPF route

Is the accumulated value from one router to the next

ITE PC v4.0


OSPF Metric Usually the actual speed of a link is different than the

default bandwidth

–This makes it imperative that the bandwidth value reflects link’s actual speed

Reason: so routing table has best path information

The show interface command will display interface’s bandwidth

-Most serial link default to 1.544Mbps

ITE PC v4.0


Basic OSPF ConfigurationModifying the Cost of a link

Both sides of a serial link should be configured with the same bandwidth

–Commands used to modify bandwidth value

Bandwidth command

–Example: Router(config-if)#bandwidthbandwidth-kbps

ip ospf cost command – allows you to directly specify interface cost

-Example:R1(config)#interface serial 0/0/0

R1(config-if)#ip ospf cost 1562

ITE PC v4.0



Modifying the Cost of the link

Difference between bandwidth command & the ip ospf cost command

–Ip ospf cost command

Sets cost to a specific value

–Bandwidth command

Link cost is calculated

ITE PC v4.0


OSPF and Multiaccess NetworksChallenges in Multiaccess Networks

OSPF defines five network types:

–Point-to-point

–Broadcast Multiaccess

–Nonbroadcast Multiaccess (NBMA)

–Point-to-multipoint

–Virtual links

ITE PC v4.0


OSPF in Multiaccess Networks

2 challenges presented by multiaccess networks

–Multiple adjacencies

–Extensive LSA flooding

ITE PC v4.0



Extensive flooding of LSAs

For every LSA sent out there must be an acknowledgement of receipt sent back to transmitting router.

consequence: lots of bandwidth consumed and chaotic traffic

ITE PC v4.0



Solution to LSA flooding issue is the use of

–Designated router (DR)

–Backup designated router (BDR)

DR & BDR selection

–Routers are elected to send & receive LSA

Sending & Receiving LSA

–DRothers send LSAs via multicast 224.0.0.6 to DR & BDR

–DR forward LSA via multicast address 224.0.0.5 to all other routers

ITE PC v4.0


OSPF in Multiaccess NetworksDR/BDR Election Process

DR/BDR elections DO NOT occur in point to point networks

ITE PC v4.0



DR/BDR elections will take place on multiaccess networks as shown below

ITE PC v4.0



Criteria for getting elected DR/BDR

1. DR: Router with the highest OSPF interface priority.

2. BDR: Router with the second highestOSPF interface priority.

3. If OSPF interface priorities are equal, the

highest router ID is used to break the tie.

ITE PC v4.0



Timing of DR/BDR Election

–Occurs as soon as 1st router has its interface enabled on multiaccess network

When a DR is elected it remains as the DR until one of the following occurs

-The DR fails.

-The OSPF process on the DR fails.

-The multiaccess interface on the DR fails.

ITE PC v4.0



Manipulating the election process

-If you want to influence the election of DR & BDR then do one of the following

Boot up the DR first, followed by the BDR, and then boot all other routers,

OR

Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers.

ITE PC v4.0


OSPF in Multiaccess NetworksOSPF Interface Priority

Manipulating the DR/BDR election process continued

–Use the ip ospf priority interface command.

–Example:Router(config-if)#ip ospf priority {0 - 255}

Priority number range 0 to 255

–0 means the router cannot become the DR or BDR

–1 is the default priority value

ITE PC v4.0


More OSPF Configuration

Redistributing an OSPF Default Route

Topology includes a link to ISP

–Router connected to ISP

Called an autonomous system border router

Used to propagate a default route

–Example of static default route

R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1

–Requires the use of the default-information originatecommand

–Example of default-information originate command

R1(config-router)#default-information originate

ITE PC v4.0



Fine-Tuning OSPF

Since link speeds are getting faster it may be necessary to change reference bandwidth values

–Do this using the auto-cost reference-bandwidth command

–Example:

R1(config-router)#auto-cost reference-bandwidth 10000

ITE PC v4.0



Fine-Tuning OSPF

Modifying OSPF timers

–Reason to modify timers

Faster detection of network failures

–Manually modifying Hello & Dead intervals

Router(config-if)#ip ospf hello-interval seconds

Router(config-if)#ip ospf dead-interval seconds

–Point to be made

Hello & Dead intervals must be the same between neighbors

ITE PC v4.0


Summary

RFC 2328 describes OSPF link state concepts and operations

OSPF Characteristics

–A commonly deployed link state routing protocol

–Employs DRs & BDRs on multi-access networks

DRs & BDRs are elected

DR & BDRs are used to transmit and receive LSAs

–Uses 5 packet types:

1: HELLO

2: DATABASE DESCRIPTION

3: LINK STATE REQUEST

4: LINK STATE UPDATE

5: LINK STATE ACKNOWLEDGEMENT

ITE PC v4.0


Summary

OSPF Characteristics

–Metric = cost

Lowest cost = best path

Configuration

–Enable OSPF on a router using the following command

R1(config)#router ospf process-id

–use the network command to define which interfaces will participate in a given OSPF process

Router(config-router)#network network-address wildcard-mask area area-id

ITE PC v4.0


Summary

Verifying OSPF configuration

–Use the following commands

show ip protocol

show ip route

show ip ospf interface

show ip ospf neighbor

ITE PC v4.0


Documents

Exploration routing chapter 01 11 full