module 2 4 distance vector routing protocols

8/2/2019 Module 2 4 Distance Vector Routing Protocols

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Module 4

Distance Vector Routing Protocols


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Identify the characteristics of distance vector routingprotocols.

Describe the network discovery process of distancevector routing protocols using Routing InformationProtocol (RIP).

Describe the processes to maintain accurate routingtables 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

Objectives

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Introduction to Distance VectorRouting Protocols


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Distance Vector Technology

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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Distance Vector Technology

Characteristics of Distance Vector routing protocols: Periodic updates Neighbors - Routing by rumor Broadcast updates 255.255.255.255 Entire routing table is included with routing update

Routers using distance vector routing are not aware of the networktopology. 5


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DV Routing Protocol Algorithms

At the core of the distance vector protocol is the algorithm . The algorithmused for the routing protocols defines the following processes:

Mechanism for sending and receiving routing information. Mechanism for calculating the best paths and installing routes in the

routing table. Mechanism for detecting and reacting to topology changes . 6


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DV Routing Protocols Characteristics

Advantages Disadvantages

Simple implementation andmaintenance

Slow convergence

Low resource requirement Limited scalability

Routing loops

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Routing Protocols Characteristics

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Network Discovery


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Cold Start

After a cold start and before the exchange of routing information, the routersinitially discover their own directly connected networks and subnet masks.

Network Discovery Cold Start

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RTA RTB RTCNetwork W Network X Network Y Network Z

Routing Table(Distance) (Vector)

Net. Hops Exit-int.

W 0


Net. Hops Exit-int.

X 0


Net. Hops Exit-int.

Y 0


Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTA

Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTB

Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTC

Exchange of Routing Information

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Net. Hops Exit-int.W 0 Y 1 RTB


Net. Hops Exit-int.X 0 W 1 RTAZ 1 RTC


Net. Hops Exit-int.Y 0 X 1 RTB



Y 2 RTA


W 2 RTBZ 2 RTB


X 2 RTC


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RTA RTB RTC

Network WNetwork X Network Y

Network Z

Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTAY 2 RTA

Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTBW 2 RTBZ 2 RTB

Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTCX 2 RTC


Net. Hops Exit-int.W 0 Y 1 RTBZ 2 RTB




Net. Hops Exit-int.Y 0 X 1 RTBW 2 RTB


Net. Hops Exit-int.W 0 Y 1 RTB




Net. Hops Exit-int.Y 0 X 1 RTB

ExistingRoutingTables

NewRoutingTables


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Net. Hops Exit-int.W 0 Y 1 RTBZ 2 RTB




Net. Hops Exit-int.Y 0 X 1 RTBW 2 RTB

RoutingTables

Convergence !


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Routing Table Maintenance


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Periodic Updates : RIPv1 & IGRP

Periodic Updates refers to the fact that a router sends the complete routingtable to its neighbors at a predefined interval .

Changes may occur for several reasons, including: Failure of a link ;Introduction of a new link ; Failure of a router ; Change of link parameters .

Periodic Updates

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Periodic Updates: RIPv1 & IGRP

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Bounded Updates: EIGRP

EIGRP uses updates that are: Non-periodic because they are not sent out on a regular basis ->Triggered by topology changes Partial updates sent only when there is a change in topology that

influences routing information. Bounded , meaning the propagation of partial updates are automatically

bounded so that only those routers that need the information areupdated.

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Triggered Updates

To speed up the convergence when there is a topology change , RIP usestriggered updates .

Triggered updates are sent when one of the following occurs: An interface changes state (up or down) A route has entered (or exited ) the " unreachable " state A route is installed in the routing table

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Routing Loops

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Definitions & Implications

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1. Network 10.4.0.0 goes down


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2. Before R3 can send an update, R2 sends an update


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3. R3 install a bad route to 10.4.0.0


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What is a Routing Loop ? A routing loop is a condition in which a packet is continuously transmitted

within a series of routers without ever reaching its intended destinationnetwork.

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4. The Network now has a loop

To


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Problem: Count to InfinityR3 install a bad route to 10.4.0.0 with a hop count of 2

R2 increases the hop count to 3 for 10.4.0.0

R1 increases the hop count to 4 for 10.4.0.0

25Each round of updates continues to increase hop count


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Solution: Setting a Maximum Metric

Once the routers " count to infinity " they mark the route as unreachable . For example:

RIP defines infinity as 16 hops - an "unreachable" metric IGRP : default maximum hop count is 100 , but can configure to 255 EIGRP : maximum hop count is 224

10.4.0.0 is unreachable. Hop count is 16

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i i i h


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Preventing Routing Loops withHolddown Timers

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P i R i L i h


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X

P i R i L i h


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P i R i L i h


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To

P i R i L i h


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P ti R ti L ith


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P ti R ti L ith


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Split Horizon Rule

The split horizon rule says that a router should not advertise a networkthrough the interface from which the update came .

Note: Split horizon can be disabled by an administrator. Under certainconditions, this has to be done to achieve the proper routing

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R2 only advertises10.2.0.0 and 10.1.0.0 to R3

R1 only advertises10.1.0.0 to R2 R2 only advertises

10.3.0.0 and 10.4.0.0 to R1

R3 only advertises10.4.0.0 to R2


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The Network is converged on poisoned route

Route Poisoning

Route poisoning is used to mark the route as unreachable in a routing updatethat is sent to other routers

Route poisoning speeds up the convergence process as the information about10.4.0.0 spreads through the network more quickly than waiting for the hopcount to reach "infinity". 35

Route Poisoning1. Network 10.4.0.0 goes down2. R3 poisons route with an infinite metric3. R3 sends triggered Poison Update to R24. 23 sends triggered Poison Update to R1


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IP & TTL

Time to Live (TTL) is an 8-bit field in the IP header that limits the number ofhops a packet can traverse through the network before it is discarded.

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Distance Vector Routing

Protocols Today


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RIP and EIGRP

The decision about which routing protocol to use in a given situationis influenced by a number of factors including:

Size of the network Compatibility between models of routers

Administrative knowledge required

Distance Vector Routing Protocol Compared

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RIP and EIGRP Features of RIP :

Supports split horizon and split horizon with poison reverse to prevents loops. Is capable of load balancing up to six equal cost paths. The default is four equal cost

paths. Easy to configure Works in a multi vendor router environment

RIPv2 introduced the following improvements to RIPv1 : Includes the subnet mask in the routing updates, making it a classless routing

protocol. Has authentication mechanism to secure routing table updates. Supports variable length subnet mask (VLSM). Uses multicast addresses instead of broadcast. Supports manual route summarization.

EIGRP features include : Triggered updates (EIGRP has no periodic updates). Use of a topology table to maintain all the routes received from neighbors (not only the

best paths). Establishment of adjacencies with neighboring routers using the EIGRP hello protocol. Support for VLSM and manual route summarization. These allow EIGRP to create

hierarchically structured large networks. Cisco proprietary protocol

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Good luck with this module!

module 2 4 distance vector routing protocols

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