CA Ex S2M04 Distance Vector Routing Protocols

8/3/2019 CA Ex S2M04 Distance Vector Routing Protocols

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Please purchase a personal license.

CCNA Ex loration version 4.0


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Objectives

Identify the characteristics of distance vector routingprotocols.

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

H c vi n m ng Bch Khoa - Website: www.bkacad.com 2

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


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Introduction to Distance Vector



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Distance Vector Routing Protocols


Dynamic routing protocols help the network administrator overcome the time-consuming and exacting process of configuring and maintaining static routes.

28 routers shown in the figure? What happens when a link goes down? Howdo you ensure that redundant paths are available?


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Routing Information Protocol (RIP): RFC 1508 Hop count is used as the metric Maximum hop count is 15 Routing updates are broadcast or multicast every 30 seconds

Interior Gateway Routing Protocol (IGRP) Proprietary protocol developed by Cisco.


Bandwidth, delay, load and reliability are used to create acomposite metric. Routing updates are broadcast every 90 seconds, IGRP is the predecessor of EIGRP and is now obsolete

Enhanced Interior Gateway Routing Protocol (EIGRP) It can perform unequal cost load balancing. It uses Diffusing Update Algorithm (DUAL) to calculate the

shortest path.

Routing updates are sent only when there is a change in thetopology.


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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 A router using a distance vector routing protocol does not have theknowledge of the entire path to a destination network. Instead the routerknows only: The direction or interface in which packets should be forwarded

The distance or how far it is to the destination network


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Characteristics of DV Routing Protocol

1. Periodic updates2. Neighbors: Routing by rumor3. Broadcast updates 255.255.255.2554. Entire routing table is included with routing update


Animation 4.1.2.2


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Routing Protocol Algorithms

Animation 4.1.3



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Routing Protocol Characteristics

Criteria used to compare routing protocols includes:1. Time to convergence


. 3. Classless (use of VLSM) or Classful4. Resource usage5. Implementation & maintenance


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Routing Protocol Characteristics



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Animation 4.1.4.2


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Network Discovery



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Cold Start

Router initial start upAfter a cold start and before the exchange of routinginformation, the routers initially discover their own directlyconnected networks and subnet masks.

Animation 4.2.1



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Exchange of Routing Information

Initial Exchange: If a routing protocol is configured then

Routers will exchange routing information Routing updates received from other routers

Router checks update for new informationIf there is new information:

Animation 4.2.2


Metric is updated New information is stored in routing table

Exchange of Routing Information Router convergence is reached when

All routing tables in the network contain the same networkinformation

Routers continue to exchange routing information If no new information is found then Convergence is reached


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Initial Exchange of Routing Information

10.1.0.0 10.2.0.0 10.3.0.0 10.4.0.0


10.1.0.0 0

Routing Table

10.2.0.0 0

10.2.0.0 0

Routing Table

10.3.0.0 0

10.3.0.0 0

Routing Table

10.4.0.0 0

10.3.0.0 1 10.1.0.0 1 10.2.0.0 1

10.4.0.0 1

10.4.0.0 2 10.1.0.0 2


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Convergence

Convergence must bereached before a network isconsidered completelyoperable

Speed of achieving

Animation 4.2.4


convergence consists of 2interdependent categories Speed of broadcasting

routing informationupdate

Speed of calculatingroutes


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Routing Table Maintenance



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Periodic Updates : RIPv1 & IGRP

Periodic updates are time intervals in which a router sends out itsentire routing table.

Changes may occur for several reasons, including:

Failure of a link Introduction of a new link Failure of a router Change of link parameters



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Periodic Updates : RIPv1 & IGRP

RIP uses 4 timers Update timer 30s Invalid timer 180s Holddown timer 180s Flush timer 240s



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Bounded Updates: EIGRP

EIRPG routing updates are Partial updates Triggered by topology changes

Bounded: meaning the propagation of partial updates are automaticallybounded so that only those routers that need the information areupdated.

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 Animation 4.3.3



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Random Jitter

Synchronized updates A condition where multiple

routers on multi access LANsegments transmit routingupdates at the same time.Problems with s nchronized u dates


Bandwidth consumption Packet collisions

Solution to problems with synchronized updatesUsed of random variable called RIP_JITTER, 0% to 15% of thespecified update interval ( 25 to 30 seconds for the default 30-second interval)

Animation 4.3.4


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Routing Loop



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Definitions & Implications

Routing loops may be caused by:1. Incorrectly configured static routes

2. Incorrectly configured route redistribution3. Slow convergence4. Incorrectly configured discard routes


Routing loops can create the following conditions: 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

Animation 4.4.1.1


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The Solutions

These mechanisms include:1. Defining a maximum metric to prevent

count to infinity2. Holddown timers


3. Split horizon4. Route poisoning or poison reverse5. Triggered updates


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Definitions & Implications

Lab 4.4.1.2



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Count to Infinity

This is a routing loop whereby packets bounce infinitelyaround a network.

H c vi n m ng Bch Khoa - Website: www.bkacad.com 28Animation 4.4.2.1


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Setting a maximum

Distance Vector routing protocols set a specifiedmetric value to indicate infinity

Once a router counts to infinity it marks the routeas unreachable



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Setting a maximum



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Preventing Routing Loops with Holddown Timers

Network is unstable: a interface resets as up , then down ,then up again in rapid succession. The route is flapping .

Using triggered updates, the routers might react too quicklyand unknowingly create a routing loop.

A routing loop could also be created by a periodic updatethat is sent by the routers during the instability.


o own t mers prevent rout ng oops rom e ng createby these conditions. Holddown timers also help prevent thecount to infinity condition.

Holddown timers allow a router to not accept any changesto 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.


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Holddown timers

Holddown timers work in the following way:1. A router receives an update from a neighbor indicating that a network

that previously was accessible is now no longer accessible.



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Holddown timers

2. The router marks the network as possibly down and startsthe holddown timer.



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Holddown timers

3. If an update with a better metric for that network is received from anyneighboring router during the holddown period, the network isreinstated and the holddown timer is removed.



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Holddown timers

4. If an update from any other neighbor is received during the holddownperiod with the same or worse metric for that network, that update isignored. Thus, more time is allowed for the information about thechange to be propagated.



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Holddown timers

5. Routers still forward packets to destination networks that are markedas possibly down. This allows the router to overcome any issuesassociated with intermittent connectivity. If the destination networktruly is unavailable and the packets are forwarded, black hole routing

is created and lasts until the holddown timer expires.



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Extra: RIP Timers

Update 30 secs Update and Invalid timers are then reset.

Invalid 180 secs - Invalid timer expires, route nowunreachable but still in routing table until flush timer

expires. Holddown timer of 180 sec begins.

Holddown 180 secs Holddown timerexpires, will now accept a poorer route to

Update timer : how frequently routing update messages should be sent

Invalid timer : how long a router should wait in the absence of routing-update messagesabout a specific route before declaring that route invalid (unreachable), but still in therouting table

Holddown timer : specifies the amount of time for which information about poorer routesare ignored.

Flush timer : how much time should pass before a route is flushed from the routing table

.

Flush 240 secs Route will now beremoved from therouting table.


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Split Horizon Rule

Split Horizon rule: A router should not advertise a network through the

interface from which the update came.

H c vi n m ng Bch Khoa - Website: www.bkacad.com 384.4.5.1


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Split horizon with Poison Reverse or Route Poisoning

Route poisoning is used tomark the route as unreachablein a routing update that is sent

to other routers. Split horizon with poisonreverse: The rule states that once a


router earns o anunreachable route throughan interface, advertise it asunreachable back throughthe same interface

Animation 4.4.6.1 Animation 4.4.6.2


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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 anetwork



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IP & TTL

How the TTL field works TTL field contains a numeric value

The numeric value is decreased by one by every routeron the route to the destination. If numeric value reaches 0 then Packet is discarded



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Distance Vector Routing Protocols today



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RIP and EIGRP

Factors used to determine whether to use RIP or EIGRPinclude Network size Compatibility between models of routers Administrative knowledge



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RIP

RIP: Supports split horizon & split horizon with poison reverse Capable of load balancing Easy to configure Works in a multi vendor router environment


Includes the subnet mask in the routing updates, making it aclassless routing protocol.

Has authentication mechanism to secure routing tableupdates.

Supports variable length subnet mask (VLSM). Uses multicast addresses instead of broadcast. Supports manual route summarization.

EIGRP


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EIGRP

Features: 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

Di V R i P l C d


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Distance Vector Routing Protocols Compared


Animation 4.5.1.1

S


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Summary



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Documents

CA Ex S2M04 Distance Vector Routing Protocols