CA_Ex_S2M04_Distance Vector Routing Protocols.ppt [Compatibility Mode]

8/3/2019 CA_Ex_S2M04_Distance Vector Routing Protocols.ppt [Compatibility Mode]

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Chapter 4 Distance Vector Routing Protocols

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).

Hc vin mng 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 andexplain the implications for router performance.

Recognize that distance vector routing protocols are inuse today


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Introduction to Distance Vector



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Distance Vector Routing Protocols


28 routers shown in the figure


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Dynamic routing protocols help thenetwork administrator overcome thetime-consuming and exacting process


routes.

What happens when a link goes down? How do you ensure that redundant

paths are available?


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Routing Information Protocol(RIP): RFC 1508

Hop count is used as the metric


Routing updates are broadcast

(ver1) or multicast (ver2) every30 seconds


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Interior Gateway Routing Protocol (IGRP)

Proprietary protocol developed by Cisco.

Bandwidth, delay, load and reliabilityare used to create a composite metric.


out ng up ates are roa cast everyseconds,

IGRP is the predecessor of EIGRP and

is now obsolete


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Enhanced Interior Gateway RoutingProtocol (EIGRP)

It can perform unequal cost loadbalancing.


It uses Diffusing Update Algorithm(DUAL) to calculate the shortestpath.

Routing updates are sent only whenthere is a change in the topology.


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Distance Vector Technology



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Distance Vector Technology

The Meaning of Distance Vector: a router using distancevector routing protocols knows 2 things:

1. Distance to final destination2. Vector or direction traffic should be directed

Routers using distance vector routing are not aware of


the network topology. The router knows only:

1. The direction or interface in which packets shouldbe forwarded

2. The distance or how far it is to the destinationnetwork


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Characteristics of DV Routing Protocol

1.Periodic updates

2.Neighbors: Routing by rumor


. 255.255.255.255

4.Entire routing table isincluded with routing update


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Characteristics of DV Routing Protocol


Animation 4.1.2.2


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Routing Protocol Algorithms

The algorithm used for the routing

protocols defines the following processes:1. Mechanism for sending andreceiving routing information.


Animation 4.1.3

2. Mechanism for calculating the bestpaths and installing routes in therouting table.

3. Mechanism for detecting andreacting to topology changes.


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Routing Protocol Characteristics

Criteria used to compare routing protocols

includes:1. Time to convergence


3. Classless (use of VLSM) or Classful

4. Resource usage

5. 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 up After a cold start and before the exchange of

routing information, the routers initiallydiscover their own directly connected networksand subnet masks.

Animation 4.2.1.1



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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.Routerchecks update for new information. If there is newinformation:

Animation 4.2.2.1


1. Metric is updated2. New information is stored in routing table

Router convergence is reached when all routing tables inthe network contain the same network information

Routers continue to exchange routing information.

If no new information is found then Convergence isreached


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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 be reached before anetwork is considered completely operable

Speed of achieving convergence consists of


1. Speed of broadcasting routinginformation update

2. Speed of calculating routes

Animation 4.2.4.1


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Routing Table Maintenance



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Periodic Updates : RIPv1 & IGRP



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Periodic updates are time intervals inwhich a router sends out its entire routing

table. Changes may occur for several reasons,

includin :


1. Failure of a link2. Introduction of a new link

3. Failure of a router4. Change of link parameters


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RIP uses 4 timersUpdate timer 30s


Holddown timer 180s

Flush timer 240s


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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 timerexpires. Holddown timer of 180 sec begins.

Holddown 180 secs Holddown timerexpires, will now accept a poorer route tosame network. Still in routing table

Flush 240 secs Route will now be

removed from therouting table.


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Invalid Timer. If an update has not been received to refreshan existing route after 180 seconds (the default), the route ismarked as invalid by setting the metric to 16. The route is

retained in the routing table until the flush timer expires.

Flush Timer. By default, the flush timer is set for 240seconds, which is 60 seconds longer than the invalid timer.When the flush timer expires, the route is removed from therouting table.

Holddown Timer. This timer stabilizes routing informationand helps prevent routing loops during periods when the

topology is converging on new information. Once a route ismarked as unreachable, it must stay in holddown long enoughfor all routers in the topology to learn about the unreachablenetwork. By default, the holddown timer is set for 180

seconds.Hc vin mng Bch Khoa - Website: www.bkacad.com 28


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Bounded Updates: EIGRP

EIRPG routing updates are

1. Partial updates2. Triggered by topology changes3. Bounded: meaning the propagation of


partial updates are automaticallybounded so that only those routers thatneed the information are updated.

4. Non periodic


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Triggered Updates

Conditions in which triggered updates are sent:

1. An interface changes state

2. A Route becomes unreachable3. A Route is placed in routing table

Hc vin mng Bch Khoa - Website: www.bkacad.com 30Animation 4.3.3


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Triggered Updates

Two problems:

1. Packets containing the update message

can be dropped or corrupted by some linkin the network.

2. The tri ered u dates do not ha en


instantaneously. It is possible that a routerthat has not yet received the triggeredupdate will issue a regular update at just

the wrong time, causing the bad route tobe reinserted in a neighbor that hadalready received the triggered update.


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Random Jitter

Synchronized updates: a condition wheremultiple routers on multi access LAN segments

transmit routing updates at the same time. Problems:


Packet collisions

Solution:

used of random variable called RIP_JITTER,0% to 15% of the specified update interval (25to 30 seconds for the default 30-secondinterval)


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Random Jitter

Animation 4.3.4.1



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Routing Loop



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Definitions & Implications

Routing loop is a condition inwhich a packet is continuously


routers without ever reaching itsdestination.


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Routing loops may be caused by:

1.Incorrectly configured static routes


redistribution3.Slow convergence

4.Incorrectly configured discard routes

Animation 4.4.1.1


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Routing loops can create the following

conditions:1.Excess use of bandwidth


. resources may e stra ne3.Network convergence is degraded4.Routing updates may be lost or not

processed in a timely manner

Animation 4.4.1.1


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The Solutions

1.Defining a maximum metric toprevent count to infinity

2.Holddown timers


.Split horizon4.Route poisoning or poisonreverse

5.Triggered updates


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Lab 4.4.1.2



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Count to Infinity

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

Hc vin mng Bch Khoa - Website: www.bkacad.com 40Animation 4.4.2.1

S


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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 theroute as unreachable


S i i


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Setting a maximum


P i R i L i h H ldd Ti


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Preventing Routing Loops with Holddown Timers

Network is unstable: a interface resets as

up, then down, then up again in rapidsuccession. The route is flapping.


,

react too quickly and unknowingly create arouting loop.

A routing loop could also be created by aperiodic update that is sent by the routersduring the instability.

P ti R ti L ith H ldd Ti


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Preventing Routing Loops with Holddown Timers

Holddown timers allow a router to not acceptany changes to a route for a specified period

of time Holddown timers prevent routing loops from


.

Holddown timers also help prevent the countto infinity condition.

Point of using holddown timers: Allowsrouting updates to propagate throughnetwork with the most current information.

Holddown timers


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Holddown timers

1. A router receives an update from a neighbor indicatingthat a network that previously was accessible is now nolonger accessible.


Holddown timers


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Holddown timers

2. The router marks the network as possibly downand starts the holddown timer.


Holddown timers


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Holddown timers

3. If an update with a better metric for that network isreceived from any neighboring router during the holddownperiod, the network is reinstated and the holddown timer is

removed.


Holddown timers


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Holddown timers

4. If an update from any other neighbor is received duringthe holddown period with the same or worse metric forthat network, that update is ignored. Thus, more time is

allowed for the information about the change to bepropagated.


Holddown timers


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Holddown timers

5. Routers still forward packets to destination networks thatare marked as possibly down. This allows the router toovercome any issues associated with intermittentconnectivity. If the destination network truly is unavailable

and the packets are forwarded, black hole routing iscreated and lasts until the holddown timer expires.


Split Horizon Rule


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Split Horizon Rule

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

Animation 4.4.5.1


Split horizon with Poison Reverse or Route Poisoning


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Split horizon with Poison Reverse or Route Poisoning

Route poisoning is used to mark the route asunreachable in a routing update that is sent to

other routers. 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 thesame interface

Animation 4.4.6.1 Animation 4.4.6.2

IP & TTL


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IP & TTL

Purpose of the TTL fieldThe TTL field is found in an IP header and is used

to prevent packets from endlessly traveling on anetwork


IP & TTL


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IP & TTL

How the TTL field worksTTL 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


RIP and EIGRP


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RIP and EIGRP

Factors used to determine whetherto use RIP or EIGRP include

1. Network size


. ompa y e weenmodels of routers

3. Administrative knowledge

RIP


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RIP

Supports split horizon & split horizonwith poison reverse


Easy to configure Works in a multi vendor router

environment

RIP


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RIPv2 introduced the following improvements toRIPv1:

1. Includes the subnet mask in the routingupdates, making it a classless routing protocol.2. Has authentication mechanism to secure


routing table updates.

3. Supports variable length subnet mask (VLSM).4. Uses multicast addresses instead of broadcast.

5. Supports manual route summarization.

EIGRP


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1. Triggered updates

2. EIGRP hello protocol used to establishneighbor adjacencies


4. Use of topology table to maintain allroutes

5. Classless distance vector routingprotocol

6. Cisco proprietary protocol

Distance Vector Routing Protocols Compared


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g p


Animation 4.5.1.1

Summary


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y


Q&A


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Three routers running a distance-vectorrouting protocol lost all power, including

the battery backups. When the routersreload, what will happen?

They will send updates that includeonly directly connected routes to theirdirectly connected neighbors.


Q&A


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What does the RIP holddown timer do?instructs routers to ignore updates,

for a specified time or event, aboutossible inaccessible routes


Q&A


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Which of the following methods doessplit horizon use to reduce incorrect

routing information?

Information learned from one source

is not distributed back to that source


Q&A


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Which of the following can exist in adistance vector network that has not

converged?

routin loo s

inconsistent traffic forwardinginconsistent routing table entries


Q&A


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If all routers are using RIP, how many

rounds of updates will occur before allrouters know all networks?



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CA_Ex_S2M04_Distance Vector Routing Protocols.ppt [Compatibility Mode]