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Bridges, routers, & broutersUnit objectives Discuss basic internetworking concepts Describe the functions of bridges,

switches and routers, describe routing protocols

Discuss Windows 2000/Server 2003 routing configuration

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Topic A Topic A: Introduction to internetworking Topic B: Introducing bridges, routers and

switches

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Internetworking

Can be defined as the technology and devices by which computers can communicate across differing types of networks

Depends on:– The number of computers on a cable

segment– The route data has to take to get to its

destination

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Internetworking devices

At the Data Link level, “Switches” are more appropriate than “Bridges”, which are all but obsolete.

Also note that “Gateways” exist at all seven OSI layers.

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Internetworking devices

Repeaters (and Hubs! – no one uses repeaters any longer. A hub is really a “multiport repeater”.

Bridges (and Switches, &%$@!!!) (as above, a switch is really a “multiport bridge”.)

Routers Gateways

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Segments and backbones

A segment is the portion of the network on either side of two network transmission devices – normally, this will be a router.

A backbone is a high-speed network link connecting only segments

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Segments connected to a backbone

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The role of the MAC address Is used to keep track of where the data packet

is going next on its way to a destination. In a “frame” that exists at the data link layer, the

frame header has a source and destination MAC address.

It also encapsulates a “packet” from the Network layer that contains, in its header, a source and destination IP address.

The destination IP address is always the final address of the frame, but the destination MAC address is the address of the “next hop.”– So, the MAC address changes every time it passes

through a router, but an IP address never does.

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Repeaters (and Hubs) Repeaters (and hubs) operate at the

Physical layer of the OSI Model Repeaters connect network segments of

similar media Problem areas that are not addressed by

repeaters– Signal quality– Time delays– Network traffic– Node limitations

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Activity A-1 - Page 6-6

Discussing internetworking basics

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Topic B

Topic A: Internetworking Topic B: Introducing bridges and

routers

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Bridges and routers Provide a way of segmenting network traffic and

connecting different LAN types Careful planning and proper implementation of

routers help you to build an efficient communications environment

The book discusses “brouters”, which combine the functions of bridges and routers. These are obsolete, and were never very popular in the first place. “Layer 3 switches”, on the other hand, do the work of switches – which are layer 2 devices, just as bridges are, as well as routers – which are layer 3 devices. They are very popular – Cisco 3550, 3560, etc. You find them in wiring closets, replacing routers, and sitting on top of several switches, which provide all the connections to the various devices such as workstations, printers, etc.

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ILTBridges

Read the physical (MAC) address of devices on a network and filter information before passing it to another network segment. A bridge divides a network into 2 “segments”, and “filtering” is a decision process in which the bridge looks at a MAC address of a packet and decides whether to send it to the other segment or to simply drop the packet because it is destined for the same segment from which it came and will therefore be picked up by the proper device without any action by the bridge whatsoever.

Bridges are obsolete and have been replaced by switches, which do the same things as bridges except they use hardware rather than software and have multiple ports instead of just two.

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Collision Domains with Bridge

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Heterogeneous (translating) bridges Interconnects different types of networks, such

as Ethernet and Token Ring.– The reason they can do this is because bridges (i.e.,

and switches) operate by dealing with the physical, or MAC addresses, found in the MAC sublayer of the Data Link layer.

– A MAC address is the same, whether it exists on a token ring or on an ethernet network. These 2 topologies also share the LLC sublayer of the Data Link layer. 802.3 is the IEEE’s ethernet implementation, and 802.5 is it’s Token Ring. But, the DIX version of ethernet is normally not compatible with the IEEE’s token ring, because DIX uses its own version of ethernet which never broke up the MAC and LLC sublayers! Good diagram on this.

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Encapsulating bridge Packages (encapsulates) frames of one

format into the format of another. This way, the frame is not read until it

reaches its final destination, so only the format of the encapsulating technology must be compatible.

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Routing management for bridges

Eliminates the possibility of duplicate frames that might be generated by having segments with multiple links that form loops in a bridged network.

Right idea, wrong choice of words. It is “switching” or “looping” management, but not “routing”. Routing is a layer 3 function and has its own rules governing traffic. See ff.

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Flow control in a bridge Is necessary to know the relative capacity of

each of the various bridge segments. Once you know the capacity, you can create

rules to govern the rate at which data can be sent and the mechanism for adjusting that rate.

Is necessary to make sure that segments with multiple links do not reproduce and distribute the same information.

The problem to be avoided is “Loops” – bridging loops and routing loops.

With bridges and switches, the “Spanning Tree Algorithm” prevents loops by ensuring that only one path exists between any two points.

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The book describes the “Spanning Tree Routing Algorithm.” This is a very bad choice of words, since there are indeed routing algorithms, but they exist at layer 3, with routers, not layer 2 with switches with which we are concerned with in this section – very misleading.

Also, the books continues to speak of bridges implementing this algorithm. Switches implement Spanning Tree. A bridge cannot shut down redundant ports because it only has two ports in the first place! See the Spanning Tree ff.

Flow control in a bridge

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20

Solution: Spanning Trees Ensure the topology has no loops

– Avoid using some of the links when flooding– … to avoid forming a loop

Spanning tree– Sub-graph that covers all vertices but contains no

cycles– Links not in the spanning tree do not forward frames

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21

Constructing a Spanning Tree Elect a root

– The switch with the smallest identifier

Each switch identifies if its interface is on the shortest path from the root– And it exclude from the tree if not– Also exclude from tree if same

distance,but with a higher identifier

Message Format: (Y, d, X) i.e., (Root-Distance-Node)– From node X – Claiming Y as root– Distance is d

root

One hop

Three hops

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22

Steps in Spanning Tree Algorithm

Initially, every switch announces itself as the root– Example: switch X announces (X, 0, X)

Switches update their view of the root– Upon receiving a message, check the root id– If the new id is smaller, start viewing that switch as root

Switches compute their distance from the root– Add 1 to the distance received from a neighbor– Identify interfaces not on a shortest path to the root and

exclude those ports from the spanning tree

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23

Switch # 4’s Viewpoint Switch #4 thinks it is the root

– Sends (4, 0, 4) message to 2 and 7 Switch #4 hears from #2

– Receives (2, 0, 2) message from 2– … and thinks that #2 is the root (lower id)– And realizes it is just one hop away

Switch #4 hears from #7– Receives (2, 1, 7) from 7

(i.e., 2 is the root, it’s 1 hop from “me”, I’m node 7.)

– And realizes this is a longer path because it adds 1 to the path from 7 – 2, wh/is

already 1 hop, so 4-7-2 = 2 hops

– So, prefers its own one-hop path (4-2 =1hop)

– And removes 4-7 link from the tree.

1

2

3

4

5

67

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24

Robust Spanning Tree Algorithm Algorithm must react to failures

– Failure of the root node Need to elect a new root, with the next lowest

identifier

– Failure of other switches and links Need to recompute the spanning tree

Root switch continues sending messages– Periodically reannouncing itself as the root (1, 0,

1)– Other switches continue forwarding messages

Detecting failures through timeout Switch waits to hear from others

– Eventually times out and claims to be the root

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A learning (transparent) bridge Automatically identifies devices on the segments

it connects. Listens to each of the attached cable

segments and creates a table of addresses originating on each segment.

Both bridges and switches create tables. When a switch receives a frame, it makes a note of the port on which it entered and the MAC address of the device with the frame. Then, when the switch receives a frame destined for the device with the MAC address that came in on the port in question, it knows which port it should exit on, to get to that device.

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Learning bridge For a more detailed view of how a

switching table is created, and how a switch or bridge learns whether to drop or forward a frame, and where to forwards it if it is not dropped, see the Cisco Press pdf document, pages 8 and 9.

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Simple Switch/Router Table

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Local and remote bridges Local bridge has a LAN link directly

attached on each side A “remote bridge link” is a local

network across a wide area segment This means you have one network, but it

exists across a wide, geographic area. This is a fairly recent phenomenon. In the past, if you had a geographic gulf, you put in a router and created two networks. But a single network works faster and is more efficient. With the newer solutions for cabling and data transfer, wide area switching is becoming more prevalent.

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Layer 2 switches

Is a more modern term for multiport bridge (i.e., a switch. More modern, like since 1983!)

Operate at the data link layer of the OSI model

Implement advanced filtering techniques to optimize performance known as Virtual LAN (VLAN) features

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Activity B-1 - page 6-13

Identifying types of bridges and switches

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Routers

Are used to segment an extended internetwork into manageable, logical subnets

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About routers Early routers supported a single protocol. Today, multiple protocol routers might

support 15 to 20 protocols simultaneously. A router has significantly greater overhead

than a switch, so they are slower; i.e., they must not only wait while a switch examines the MAC address, but then must examine the logical, IP address as well. And both the switching table – MAC address to port, as well as the routing table – IP address to router interface, must be populated.

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Router features

Processor/memory/storage Physical interfaces (ports) supported Protocols supported Configuration/management

(open/proprietary)

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Key points about routers Connect two or more subnetworks, which are

defined by the router interfaces at each end. Might be configured to support one or more

protocols Only process packets specifically addressed

to them as a destination, i.e., ip address. Packets destined for a locally connected

subnetwork are passed to that network Packets destined for a remote subnetwork are

passed to the next router in the path A router that exists in the same subnet as a

host can be configured as a default gateway.

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A routing table is normally populated dynamically, when the routing protocol takes an incoming packet and places the source ip address of the packet into a routing table row headed by the router interface on which the packet entered the router.

An administrator can manually enter routes into the router, which ensures that packets will take that route to a destination.

When choosing between alternative routes, a router relies on various factors. The reliability of a route is the key in choosing a route. A static route is the most reliable route there is, next to being an interface that actually exists on the router.

Key points about routers - cont

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Routers with static routes Note that the book reads “static or dynamic

“routers”. A router is neither static nor dynamic – only routes are static or dynamic, and the routing protocols used to route traffic create dynamic routes, that change when a current route goes down, or when a better route is found.

A route that is manually configured and that the router must follow when sending out a packet, is a static route.

Static routes are more difficult to manage and less efficient than their dynamic counterparts for several reasons– Manual configuration– Manual updates– Changing environments

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Routers with dynamic routes Dynamic routes use an Interior Gateway

Protocol (IGP) to communicate with each other

The two most common Interior Gateway protocols are:– Routing Information Protocol (RIP) – a

“distance vector” routing protocol that is now obsolete. RIPv.2 often replaces RIP, as does EIGRP or IGRP, both proprietary Cisco protocols. Uses the Bellman-Ford algorithm.

– Open Shortest Path First (OSPF) – a “link state” routing protocol, based on the Dykstra, or the “Open Shortest Path First” protocol.

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Routing tables Routers using static and dynamic routes use

routing tables to pass packets to subnetworks. A routing table matches an incoming packet’s

source ip address with the router’s interface on which it entered, in a spreadsheet layout of column and row.

A routing protocol will populate this table dynamically, as soon as the router is turned on. This is “convergence”. Updates are made dynamically, at intervals, depending on the metric a protocol uses to measure the value of a route.

An administrator will create, and later update routes by manually entering the source and destination IP address as well as other factors, both when the routes are created, as well as when changes occur in the internetwork that require a change of route.

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Sample routing table

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Routing examples

Some specific situations are handled as follows– Local destination

The packet will be addressed to the destination host and other systems, including routers, will ignore the packet

– Remote destination, next hop known The source host will place the IP address

for the next router as the immediate destination

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Routing examples– Remote destination, next hop unknown

The source host will place the IP address for the default gateway as the immediate destination

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Brouters

Operate at both the network layer for routable protocols and at the Data Link layer for non-routable protocols

Handle both routable and non-routable features by acting as routers for routable protocols and bridges for non-routable protocols

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Bridges vs. routers

Routers should be given preference over bridges when designing and configuring WANs

Bridges, by design, can escalate a transient reliability problem into a serious network failure

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Physical & Data Link Layers with Ethernet and other Layer 2 topos

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Old and New(er) Ethernet Packet Types.

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IP Header

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IP Header Fields Explained 1. Version - The version is a binary number that is four bits long. It indicates which version of IP

is being used. Currently we are using IP version four, although IP version six will soon make an impact on the networking world.

2. IHL (Internet Header Length) - The IHL simply measures the length of the IP header in 32-bit words. The minimum header length is five 32-bit words.

3. Type of Service - This field is for specifying special routing information. This field in particular relates to Quality of Service technologies quite well. Essentially, the purpose of this 8-bit field is to prioritize datagrams that are waiting to pass through a router.

4. Total Length - This 16-bit field includes the length of the IP datagram. This length includes the IP header and also the data itself.

5. Identification - This is a 16-bit field that acts as a means of organizing chunks of data. If a message is too large to fit in one data packet, it is split up and all of its child packets are given the same identification number. This is handy to ensure data is rebuilt on the receiving end properly.

6. Flags - This field signifies fragmentation options- such as whether or not fragments are allowed. The Flags field also has capability to tell the receiving source that more fragments are on the way, if enabled. This is done with the MF flag, also known as the more fragments flag.

7. Fragment Offset - This is a 13-bit field that assigns a number value to each fragment. The receiving computer will then use these numbers to reassemble the data correctly. Obviously this is only applicable if fragments are allowed.

8. Time to Live - This is often known as TTL. It is a field that indicates how many hops a data packet should go through before it is discarded. Every successful pass through a router, known as a hop, decrements this field by one. When it reaches zero, it is discarded.

9. Protocol - This 8-bit field indicates which protocol should be used to receive the data. Some of the more popular protocols such as TCP and UDP are identified by the numbers 6 and 17 respectively.

10. Header Checksum - This 16-bit field holds a calculated value that is used to verify that the header is still valid. Each time a packet travels through a router this value is recalculated to ensure the header is still indeed valid.

11. Destination IP Address - This 32-bit field holds the IP address of the receiving computer. It is used to route the packet and to make sure that only the computer with the IP address in this field obtains the packets.

12. Source IP Address - This 32-bit field holds the IP address of the sending computer. It is used to verify correct delivery, and will also be the return address in case an error occurs.

13. IP Options - This field can hold a fair number of optional settings. These settings are primarily used for testing and security purposes. Although clever settings such as keeping timestamp data from each router hop may seem handy, it will actually degrade speed more often than not.

14. Padding - Since the IP options field varies in length depending on the configuration, we need to have this field set to occupy left over bits. This is because the header needs to be ended after a 32-bit word: no more, no less.

15. Data - This is fairly self explanatory- it is simply the data that is being sent.

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Right: TCP header model Left: Capture of IP and

TCP headers using a Packet Sniffer.

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– Source and destination port :These fields identify the local endpoint of the connection. Each host may decide for itself how to allocate its own ports starting at 1024. The source and destination socket numbers together identify the connection.

Sequence and ACK number : This field is used to give a sequence number to each and every byte transferred. This has an advantage over giving the sequence numbers to every packet because data of many small packets can be combined into one at the time of retransmission, if needed. The ACK signifies the next byte expected from the source and not the last byte received. The ACKs are cumulative instead of selective.Sequence number space is as large as 32-bit although 17 bits would have been enough if the packets were delivered in order. If packets reach in order, then according to the following formula: (sender's window size) + (receiver's window size) < (sequence number space)the sequence number space should be 17-bits. But packets may take different routes and reach out of order. So, we need a larger sequence number space. And for optimisation, this is 32-bits.

Header length :This field tells how many 32-bit words are contained in the TCP header. This is needed because the options field is of variable length.

Flags : There are six one-bit flags. – URG : This bit indicates whether the urgent pointer field in this packet is being used. – ACK :This bit is set to indicate the ACK number field in this packet is valid. – PSH : This bit indicates PUSHed data. The receiver is requested to deliver the data to the

application upon arrival and not buffer it until a full buffer has been received. – RST : This flag is used to reset a connection that has become confused due to a host crash or

some other reason.It is also used to reject an invalid segment or refuse an attempt to open a connection. This causes an abrupt end to the connection, if it existed.

– SYN : This bit is used to establish connections. The connection request(1st packet in 3-way handshake) has SYN=1 and ACK=0. The connection reply (2nd packet in 3-way handshake) has SYN=1 and ACK=1.

– FIN : This bit is used to release a connection. It specifies that the sender has no more fresh data to transmit. However, it will retransmit any lost or delayed packet. Also, it will continue to receive data from other side. Since SYN and FIN packets have to be acknowledged, they must have a sequence number even if they do not contain any data.

Explanation of TCP header fields:

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Explanation of TCP header fields:

Window Size : Flow control in TCP is handled using a variable-size sliding window. The Window Size field tells how many bytes may be sent starting at the byte acknowledged. Sender can send the bytes with sequence number between (ACK#) to (ACK# + window size - 1) A window size of zero is legal and says that the bytes up to and including ACK# -1 have been received, but the receiver would like no more data for the moment. Permission to send can be granted later by sending a segment with the same ACK number and a nonzero Window Size field.

Checksum : This is provided for extreme reliability. It checksums the header, the data, and the conceptual pseudoheader. The pseudoheader contains the 32-bit IP address of the source and destination machines, the protocol number for TCP(6), and the byte count for the TCP segment (including the header).Including the pseudoheader in TCP checksum computation helps detect misdelivered packets, but doing so violates the protocol hierarchy since the IP addresses in it belong to the IP layer, not the TCP layer.

Urgent Pointer : Indicates a byte offset from the current sequence number at which urgent data are to be found. Urgent data continues till the end of the segment. This is not used in practice. The same effect can be had by using two TCP connections, one for transferring urgent data.

Options : Provides a way to add extra facilities not covered by the regular header. eg,

– Maximum TCP payload that sender is willing to handle. The maximum size of segment is called MSS (Maximum Segment Size). At the time of handshake, both parties inform each other about their capacity. Minimum of the two is honoured. This information is sent in the options of the SYN packets of the three way handshake.

– Window scale option can be used to increase the window size. It can be specified by telling the receiver that the window size should be interpreted by shifting it left by specified number of bits. This header option allows window size up to 230.

Data : This can be of variable size. TCP knows its size by looking at the IP size header.

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IP “Packet” Encapsulated in a Data Link Layer “Frame”

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UDP Header

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TCP port numbersTable 1 - Frequently used TCP port numbers

Port Number Process Name Description

1 TCPMUX TCP Port Service Multiplexer

5 RJE Remote Job Entry

7 ECHO Echo

9 DISCARD Discard

11 USERS Active Users

13 DAYTIME Daytime

17 Quote Quotation of the Day

19 CHARGEN Character generator

20 FTP-DATA File Transfer Protocol - Data

21 FTP File Transfer Protocol - Control

23 TELNET Telnet

25 SMTP Simple Mail Transfer Protocol

27 NSW-FE NSW User System Front End

29 MSG-ICP MSG-ICP

31 MSG-AUTH MSG Authentication

33 DSP Display Support Protocol

35   Private Print Servers

37 TIME Time

39 RLP Resource Location Protocol

41 GRAPHICS Graphics

42 NAMESERV Host Name Server

43 NICNAME Who Is

49 LOGIN Login Host Protocol

53 DOMAIN Domain Name Server

67 BOOTPS Bootstrap Protocol Server

68 BOOTPC Bootstrap Protocol Client

69 TFTP Trivial File Transfer Protocol

79 FINGER Finger

101 HOSTNAME NIC Host Name Server

102 ISO-TSAP ISO TSAP

103 X400 X.400

104 X400SND X.400 SND

105 CSNET-NS CSNET Mailbox Name Server

109 POP2 Post Office Protocol v2

110 POP3 Post Office Protocol v3

111 RPC Sun RPC Portmap

137 NETBIOS-NS NETBIOS Name Server

138 NETBIOS-DG NETBIOS Datagram Service

139 NETBIOS-SS NETBIOS Session Service

146 ISO-TP0 ISO TP0

147 ISO-IP ISO IP

150 SQL-NET SQL NET

153 SGMP SGMP

156 SQLSRV SQL Service

160 SGMP-TRAPS SGMP TRAPS

161 SNMP SNMP

162 SNMPTRAP SNMPTRAP

163 CMIP-MANAGE CMIP/TCP Manager

164 CMIP-AGENT CMIP/TCP Agent

165 XNS-Courier Xerox

179 BGP Border Gateway Protocol

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Activity B-2 - page 6-20

Discussing routers and brouters

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Understanding the routing protocols

Two basic types of routing algorithms– Distance vector algorithms– Link state algorithms

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Distance vector algorithms

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Routing Protocols

Dynamic routing using routing protocols

Purpose of routing protocols is to build a “routing table” with the best routes

Routing protocols are categorized into two types:– Distance Vector– Link State

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Routing Protocols Distance vector routing protocols are simple Generally they are easy to configure They use simple logic to determine the best

path to a given destination The term metric refers to the method or

measurement used by the routing protocol logic to determine the “best path” to a given network

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Routing Protocols A distance vector routing protocol usually uses

hop count as its metric A distance vector routing protocol is

characterized by how it communicates with other routing devices

Distance vector routing protocols use broadcasts to advertise their entire routing table to “directly connected” peer routers

A router is “directly connected” if it is at the end of a cable or some other connecting device, the other end of which is plugged into the router in question, i.e., the “directly-connected” router.

So, if I have a router with 3 interfaces, it can have 3 directly connected “neighbors.”

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Routing Protocols “Convergence” is the time it takes for a given

set of routers to learn routes to all the other routers in the “internetwork”.

Convergence describes the time it takes a set of routers to learn of a change in the network

Distance vector routing protocols generally take longer to converge than link state protocols because they use a periodic route advertisement schedule.

RIP, for example, sends it’s entire routing table to its directly connected neighbors every 30 seconds.

The next 3 slides are from another Power Point

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Routing Protocols (other PPt)

Dynamic routing uses routing protocols

Purpose of routing protocols is to build a routing table with the best routes

Routing protocols are categorized into two types:– Distance Vector– Link State

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Routing Protocols (other PPt)

Distance vector routing protocols are simple

Generally they are easy to configure They use simple logic (algorithms) to

determine the “best path” to a given destination

The term “metric” refers to the method or measurement used by the routing protocol logic to determine the best path to a given network – e.g., hops, bandwidth, latency, etc.

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Routing Protocols (other PPt) A distance vector routing protocol usually uses hop

count as its metric (RIP and RIPv.2). [IGRP – Cisco proprietary – on the other hand, uses 4 metrics and MTU, Maximum Transmission Unit, as a tie-breaker. The four metrics are Bandwidth, Distance, Latency and Reliability]. Only 2 are used at any one time, with bandwidth and delay the default metrics. The hop count is 256 max, with 100 hops the default.

A distance vector routing protocol is characterized by how it communicates with other routing devices

Distance vector routing protocols use broadcasts to advertise their entire routing table to directly connected peer routers. (With RIP, the broadcasts are every 30 seconds; with IGRP it’s every 90 seconds. This is very bandwidth-intensive and one reason that link-state routing protocols are preferred in large networks with many devices. The more devices there are, the more broadcasts will be clogging the network.)

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Routing Protocols

A routing loop occurs when routers get confused during update operations, causing frames to bounce back and forth between a set of interfaces

Two easy methods to identify routing loops:– Tracert or traceroute (TCP/IP utilities)– View the routing table and the metric

associated with the network

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Routing Protocols Prevent routing loops by using the following

software-based methods:– Split horizon – Hold-down timers – Triggered updates – Hop count limits – Poisoning

Note: the Network+ objectives don’t mention “loops” at all, nor do they require a knowledge of any of the loop-avoidance methods above. The CCNA exam requires a fairly detailed knowledge, however, so I think a brief discussion here is appropriate.

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Routing Loops Prevention First, I should note that the reason that loops

occur in the slow convergence of distance vector protocols. Loops occur when every router is not updated at close to the same time. Link State protocols almost never have loops because they converge in a very few seconds.

Split Horizon: Information cannot be sent back in the direction from which it was received.

Hold-down Timers: Prevent regular update messages from too rapidly reinstating a route that has gone down. It allows time for the down route to either come back up, or for the network to stabilize before turning to the next best route. It enforces a waiting time before changing a route that has recently changed.

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Routing Loops Prevention Triggered Updates: These go with the hold-

down timers, which start when a router gets a message that route is down. A triggered update will reset the timer under certain conditions, such as when the hold-down timer expires or when another update is received indicating a change in the status of the network. The triggered update will create a new routing table that includes the change reflected in the new update.

Maximum Hop Count: This is the classic technique, built into all distance vector protocols. It’s called “counting to infinity”. With RIP, any more than 15 hops is considered an infinite distance and the packet is dropped. With IGRP/EIGRP the max hop count is 256, although by default 100 hops is the limit.

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Routing Loops Prevention Route Poisoning: (or “poison reverse”).

This technique enters a routing table entry when a route goes down. It describes the down route as having an infinite distance from the network, thus preventing it from being advertised, at least for a time. It is used with a hold-down timer in order to limit the duration of the poisoning.

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Routing Protocols Link state routing protocols are more

intelligent than distance vector protocols The metric used by most link state protocols is

“cost”, based in turn on bandwidth allowing more complex routing configurations

Routing protocols capable of making complex decisions use a mathematical formula or algorithm for deriving the best path or route to a given network

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Routing Protocols Some link state protocols  are

capable of determining the best route to a destination network based on the following:– Delay – Bandwidth– Load – Reliability – MTU

Distance vector routing protocols, other than RIP, also use these same metrics; e.g., IGRP and EIGRP.

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Routing Protocols When more than one metric is used it is

referred to as a “composite metric” Link state protocols only send updates

when changes occur, and they only send the changes, not the entire route table– In fact, they do send the entire table, but only at

very long intervals, from one to several hours. Link state protocols use multicast and

unicast traffic instead of broadcast traffic Link state routers also develop an overall

picture of the networks available by establishing “neighbor” relationships

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RIP (v.1 and 2) (distance vector) Broadcasts a request for routing table

information from all other routers it can “see” – (it can “see” any router directly connected to one of its interfaces)

The information received is used by the router to determine the shortest path to each destination

The route information is then entered into the local routing table

The router sends a RIP broadcast every 30 seconds

The broadcast contains its known destinations and the cost (in hops) to get to each

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RIP RIP v.1 is almost never used any longer,

except in the smallest networks. First, it doesn’t “scale” well, i.e., it has a

“hop” limit of 15 hops. If it takes more than 15 hops to get to a destination, the destination is considered to be an infinite distance.

Second, RIP v.2 is a vast improvement over v.1; it’s still 15 hops max, but it can understand “variable length subnet masks” and supports “discontiguous” networks (more on these later).

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OSPF (link state)

OSPF packets are carried within IP datagrams

Link State algorithm provides several enhancements over RIP

Hierarchical topology configuration Support for large internetworks Adaptation to changing conditions Traffic or “load” balancing over

multiple paths Authentication of router table

information exchange

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ILTICMP

Is a module of IP that provides error reporting during datagram processing

A common use is passing error information between host and router

This error data provides dynamic routing table updates

The “Ping” utility uses ICMP, as do several other useful utilities.

This doesn’t belong with RIP and OSPF. Don’t be misled by their proximity in the PPt.

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Routing support in Windows

Windows 2000 Server and Windows Server 2003 support both RIP and OSPF

You have to configure routing in the Routing and Remote Access Service (RRAS)

RRAS is installed by default, but not enabled or configured

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The General tab for a configured router

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Activity B-3

Understanding routing protocols

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Unit summary

Discussed internetworking Described the functions of bridges,

routers, switches, routing protocols and Windows 2000/Server 2003 routing configuration