Routing Protocols

Page 104/21/23

• A router ‘knows’ only of networks attached to it directly – unless you configure a static route or use routing protocols

• Routing protocols share this information among immediate neighbors first, and then throughout the network

• Routers do not calculate the entire path to a destination

They select the next hop leading to that destination

Rely on the next hop router to select a further hop that gets the packet closer to its destination

A router’s routing tables contains information about the networks directly connected to it, as well as information that the router has “learned” about more distant networks (sample of a routing table)

Routing Protocols

Page 204/21/23

•Important function of a router (layer 3):

Examine incoming data packets and make path selection based on information stored in their routing tables

Router evaluates the available paths to a destination to establish the best path for routing a packet.

Most routing protocols use the shortest and best path - but by different means.

Routing Protocols (cont.)

Page 304/21/23

Characteristics of static routing:

• administrators manually configure static routes

• routes do not change, unless changed by the administrator

• are useful in environments where there is a single link in or out of the networks (know as a stub network)

• because there is only one link to get to the neighbor network, we conserve bandwidth not being used by routing protocols

Static Routing

Page 404/21/23

Static route can be defined by:

RouterA(config)# ip route <dest network> <subnet mask> < interface | next hop>

Router A Router BNetwork 200.10.10.0/24

192.16.10.1s0

(Packets arriving at Router A with network 200.10.10.0 as their destination will be sent to the serial port of the next router – that is, the next hop)

RouterA(config)# ip route 200.10.10.0 255.255.255.0 192.16.10.1

(The same thing can be accomplished by this:

Packets arriving at Router A with network 200.10.10.0 as their destination will be sent out of Router A via Serial port 0)

RouterA(config)# ip route 200.10.10.0 255.255.255.0 s0

Static Routing

Page 504/21/23

Router A Router B

Network 200.10.10.0192.16.10.1

s0


e0

Static address command for network 195.10.10.0 to get to network 200.10.10.0 is:

195.10.10.0 net


Static Routing

OR

Note that when the interface option is specified, it’s our interface ( RouterA - not the neighbor’s).

Page 604/21/23

Router A Router B

Network 200.10.10.0192.16.10.1

s0



The above example will have an administrative distance of 1 in the routing table.

The above example will have an administrative distance of 0 in the routing table.

Static Routing

RouterA(config)# show ip route

Page 704/21/23

Router A Router B

Network 200.10.10.0192.16.10.1

s0


If the network 192.16.10.0 goes down for any reason, the static address on RouterA will be removed from the routing table.

Static Routing

Page 804/21/23

Static Routing

Page 904/21/23

If router doesn’t know where to send a packet, it uses a default route:

RouterA(config)# ip route 0.0.0.0 0.0.0.0 < interface | next hop>

Router A

Internet

192.16.10.1s0



Default Route

Gateway of last resort for routers that do not have a network entry in the routing table

Packets going to any network for which I don’t have an entry in the routing table, just send them to the next hop

OR

Page 1004/21/23

Default Route on non-directly connected networks

Page 1104/21/23

To verify a static route or default route use the commands:

show running-config

Or

show ip route

To test a static route or default route use the commands:

ping

Or

traceroute

Default/Static Route

Page 1204/21/23

Page 1304/21/23

[Administrative distance / Hop count]

Examine the Show ip route command

Page 1404/21/23

Default Administrative Distances

Routed Source Default Distance

Connected 0

Static Route 1

EIGRP(internal) 90

OSPF 110

RIPv1 AND v2 120

EIGRP(external) 170

Page 1504/21/23

Routed vs. Routing:

Routed protocols are protocols are protocols used to carry the data across the networks – like those in the IP protocol (such as Telnet) and Novel lIPX

Routing protocols route routed protocols through a network. (Routers use routing protocols to talk to each other about networks & paths.

• Dynamically advertise and learn routes, determine which routes are available and which are the most efficient routes to a destination)

• IGRP (Interior Gateway Routing Protocol)

• EIGRP ( Enhanced IGRP)

• BGP (Border Gateway Protocol)

• OSPF (Open Shortest Path First)

• APPN (Advanced Peer-to-Peer Networking)

• IS-IS (Intermediate System-to Intermediate System)

• RIP (Routing Information Protocol)

Routed Vs. Routing Protocols

Page 1604/21/23

An autonomous system (AS) is a collection of networks under a common administration.

The American Registry of Internet Numbers (ARIN) assigns an identifying number to each AS.

AS is a 16 bit number.

Some routing protocols such as IGRP, EIGRP requires AS for their configurations.

Autonomous Systems

Page 1704/21/23

When complex networks have multiple interconnections, static routes are not a practical solution – they can’t react to changes in the topology

When all routers in an internetwork are operating with the same knowledge of the routes in a network is said to have converged.

Fast convergence is desirable because it reduces the period of time in which routers would continue to make incorrect routing decisions.

In routers that use dynamic routing protocols, it is important to have fast convergence because routers could make incorrect forwarding decisions until the network has fully converged.

Routing Protocols

Page 1804/21/23


Page 1904/21/23

Routing protocols are divided into 2 classes:

• interior - used for routing information within networks that are under a common network administrator

• RIP

• IGRP

• OSPF

• EIGRP

• exterior - used for routing information between networks that do not share a common administrator.

• EGP (exterior gateway protocol)

• BGP


Page 2004/21/23

Classes of Routing Protocols

• distance-vector - determines the direction & distance

• IGRP

• RIP

• link-state (also called shortest path first) re-creates the exact topology of the entire network.

• OSPF

• IS-IS

Classes of Routing Protocols

Page 2104/21/23

Distance vector routing algorithms pass periodic copies of a routing table from router to router. These regular updates are based on the routing algorithm know as ‘Bellman-Ford’ algorithm.

However distance vector algorithms do not allow a router to know the exact topology of an internetwork as each router only sees its neighbor routers.

Distance Vector Routing Protocols

Page 2204/21/23

Neighbor to neighbor ONLY


Page 2304/21/23


Page 2404/21/23

Routing table updates take place periodically.

Router to router (neighbor tells neighbor)

Distance vector – each router sends its entire routing table to neighbor table


Page 2504/21/23

Link-state algorithms are also known as Dijkstras algorithm or SPF (shortest path first) algorithm.

Link-state routing algorithms maintain a complex database of topology information. It maintains full knowledge of distant routers and how they are interconnected.

Link-state routing uses:

• Link-state Advertisements (LSAs)

Small packets of routing info that are sent between routers

• Topological database

• SPF algorithm

• Routing tables

Link-state Routing Protocol

Page 2604/21/23

Link-state makes an update only when there is a topology change.

Link-state concerns itself with:

• processor overhead

• memory requirements

• bandwidth consumption

After the initial flood, it passes small event-triggered updates to all other routers

Link-state Routing Protocol

Page 2704/21/23

A responsibility of a Routing Protocol is to build routing tables to determine optimal routing paths

They need to have a way to measure (metrics) which routes are preferred when there are multiple paths to a destination

Different routing protocols use different metrics

1.Hop count: number of routing devices the packet needs to travel to reach the destination network

2.Bandwidth: cumulative bandwidth of the links to the destination measured in kilobits per second

3.Delay: length of time (in microseconds) a packet takes from source to destination

4.Reliability: consistency of the links and paths toward the destination based on the interfaces’ error rates

5.Load: cumulative amount of congestion of the links in the direction of the destination

Routing Metrics

Page 2804/21/23

1. Hop count: number of routing devices the packet needs to travel to reach the destination network - counts router hops

2. Bandwidth: cumulative bandwidth of the links to the destination measured in kilobits per second

3. Delay: length of time (in microseconds) a packet takes from source to destination

4. Reliability: consistency of the links and paths toward the destination based on the interfaces’ error rates

5. Load: cumulative amount of congestion of the links in the direction of the destination

6. MTU: maximum frame size that is allowed to cross the links to the destination

7. Cost: arbitrary number typically based on the bandwidth of the link

Routing Metrics (cont.)

Page 2904/21/23

• The physical address (MAC) changes at each hop.

192.168.1.5601-00-A5-C3-26-6B

192.168.1.5634-7E-33-12-C9-20

192.168.1.566A-5F-0D-09-8B-AA

192.168.1.56BC-48-03-8F-FF-AA

• The protocol address (IP) always remains the same.

192.168.1.56

The Routing Process

Page 3004/21/23

Characteristics of RIP are:

• distance vector

• metric is hop count

• if the hop count is greater than 15, the packet is discarded

• is updated every 30 seconds

For RIP, each router that it has to go through to get to a network is considered a hop count. That is, if you have to go through 2 routers to get to a network, then the hop count is 2.

If the network is on the next router, then the hop count is 0. (That is a directly connected serial port.)

RIP Routing Protocol

Page 3104/21/23

IGRP:

• must be assigned an AS (autonomous system number - 16 bit number)

• Cisco proprietary

• distance-vector

• metrics

• delay

• bandwidth (1200 bps - 10 Gbps)

• reliability (1-224) (higher the number, more reliable)

• load (1-244) (higher the number, more it is under load)

•sends updates every 90 seconds• maximum hop count is 255 (default 100)

IGRP Routing Protocol

Page 3204/21/23

Characteristics of OSPF are:

• link-state routing protocol

• open standard

• uses the SPF algorithm to calculate the lowest cost to a destination

• routing updates are flooded as topology changes occur.

OSPF Routing Protocol

Page 3304/21/23

Characteristics of EIGRP are:

• distance-vector

• Cisco proprietary

• uses load balancing

• uses a combination of distance vector & link-state features

• use Diffused Update Algorithm (DUAL) to calculate the shortest path

• updates broadcast every 90 seconds

EIGRP Routing Protocol

Page 3404/21/23

Characteristics of BGP are:

• distance vector exterior routing protocol

• used between ISP and clients

• used to route internet traffic between autonomous systems

BGP Routing Protocol

Page 3504/21/23

RouterA#config t

RouterA(config)# router <routing protocol> [AS number]

RouterA(config-router)# network <networks on the routers interfaces>

Example:

RouterA(config)# router rip

RouterA(config-router)# network 192.16.10.0

Example:

RouterA(config)# router igrp 100


Configure Routing Protocols

‘advertises’ networks directly connected to it

Page 3604/21/23

RouterA# config t

s0s1

e0

192.10.10.0 net

172.120.0.0 net

210.45.20.0 net




RouterA(config)# router rip

Configure RIP Routing Protocol

Documents

Routing Protocols