ccna exp2 - chapter04 - distance vector routing protocols

8/3/2019 CCNA Exp2 - Chapter04 - Distance Vector Routing Protocols

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Chapter 4 - Distance Vector Routing ProtocolsRouting Protocols and Concepts

CCNA Exploration 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 Information

Protocol (RIP). Describe the processes to maintain accurate routing

tables 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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Distance Vector Routing Protocols

Examples of Distance Vector routing protocols:

Routing Information Protocol (RIP) RFC 1508 Hop count is used as the metric

Max hop 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 theshortest 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 protocolsknows 2 things:

Distance to final destination

Vector or direction traffic should be directed


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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



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At the core of the distance vector protocol is the algorithm. The

algorithm is used to calculate the best paths and then send thatinformation to the neighbors.

Routing Protocol Algorithm:

- Defined as a procedure for accomplishing a certain task.



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Routing Protocol Characteristics Criteria used to compare routing protocols includes

Time to convergence Scalability Resource usage Implementation & maintenance



Distance Vector Routing Protocols Comparison



Network Discovery

Router initial start up After a cold start and before the exchange of routing

information, the routers initially discover their owndirectly connected networks and subnet masks.



Network Discovery

Initial Exchange of Routing Information If a routing protocol is configured then- Routers will exchange routing information

Routing updates received from otherrouters

Router checks update for new information If there is new information:

o Metric is updated

o New information is stored in

routing table Refer to 4.2.2.1



Exchange of RoutingInformation

Router convergence isreached when

- All routing tables in the

network contain the samenetwork information

Routers continue to exchangerouting information

-If no new information is foundthen Convergence is reached

Refer to 4.2.3.1.

Network Discovery



Network Discovery

Convergence must be reachedbefore a network is consideredcompletely operable

Speed of achievingconvergence consists of 2

interdependent categories Speed of broadcasting

routing information

Speed of calculating routes



Routing Table Maintenance

Periodic Updates : RIPv1 & RIPv2

These 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




RIP uses 4 timers Update timer 30s.

Invalid timer 180s.

Holddown timer 180s.

Flush timer 240s.




Bounded Updates: EIGRP

EIRPG routing updates are Partial updates

Triggered by topology changes

Bounded: meaning the propagation of partial updates

are automatically bounded so that only those routersthat need the information are updated.

Non periodic




Triggered Updates

Conditions in which triggered updates are sent-Interface changes state.

-Route becomes unreachable.

-Route is placed in routing table.



Random Jitter Synchronized updates

A condition where multiple routers on multi access LAN segmentstransmit routing updates at the same time.

Problems with synchronized updates

Bandwidth consumption. Packet collision.

Solution to problems with synchronized updates

- Used of random variable called RIP_JITTER, 0% to 15% of thespecified update interval (25 to 30 seconds for the default 30-second

interval).




Routing Loops

Definitions & Implications Routing loops are

A condition in which apacket is continuouslytransmitted within a series of

routers without everreaching its destination.



Routing loops may be caused by:- Incorrectly configured static routes.- Incorrectly configured route redistribution.

- Slow convergence.

- Incorrectly configured discard routes. Routing loops can create the following issues

- Excess use of bandwidth.

- CPU resources may be strained.

- Network convergence is degraded.- Routing updates may be lost or not processed in

timely manner.

Routing Loops


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Routing Loop Issues

Routing Loop Example

Assume for the remainder of this example that Router Cs preferred path to

network 1 is by way of Router B. Router Cs routing table has a distance of 3 to network 1 via Router B.



Routing Loop Issues

Network 1 Fails

Router E sends an update to Router A.

Router A stops routing packets to network 1.

But Routers B, C, and D continue to do so because they have not yet beeninformed about the failure.

Router A sends out its update.

Routers B and D stop routing to network1, (via Router A).

However, Router C is still not updated.

To router C, network 1 is still reachable via router B.



Routing Loop Issues

Router C sends a periodic update to Router D

Router C sends a periodic update to Router D indicating a path to network 1(by way) of via Router B. (4 hops).

Router Ds Routing Table information for Network 1

Current path to Network 1 = Unreachable (down) Information from Router C: Network 1 : 4 hops by way of Router C

Normally, RouterD ignores this routing information because it usually has abetter route, 2 hops, via Router A, but this route is now down.

Router D changes its routing table to reflect this (good) better, but incorrectinformation, Network 1 by way of Router C (4 hops)

Router D propagates the information to Router A.



Routing Loop Issues

Routers A changes its routing table

Router A adds new route to its routing table, get to Network 1 by way of RouterD (5 hops).

Propagates the information to Routers B and E.

Router B (and Router E) change their routing tables

Router B now believes it can get to Network 1 by way of Router A (6 hops).

Wow! I was about to tell Router C that Network 1 was down via Router B, butnow I have new information!

Propagates the incorrect information to Router C.



Routing Loop Issues

Router C changes its routing table

Router C still believesit can get to Network 1 by way of Router B (7 hops).

Of course now it believes it is 7 hops instead of 3.

Propagates the newer but still incorrect information to Router D.

Here we go again!

Data packets destined for Network 1 get caught in a routing loop, from RoutersA to D to C to B to A to D etc.

As routing updates continue between the routers, the hop count gets greater

to infinity? (Not quite we will see in a moment.)


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Prevent routing loops

Several remedies to have been added to distance-vectoralgorithms to help prevent routing loops including:

Defining a maximum metric

Hold-down timers Split horizon

Route poisoning or poison reverse

Triggered updates



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Defining a Maximum

Problem: Count to infinity

Solution: Defining a Maximum

Distance vector routing algorithms are self-correcting, but a routing loopproblem can require a count to infinity.

To avoid this prolonged problem, distance vector protocols define infinity as aspecific maximum number.

This number refers to a routing metric which may simply be the hop count. When the metric value exceeds the maximum value, and as each router

receives this maximum metric, the network is then considered unreachable.


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Preventing loops with holddown timers Holddown timers allow a router to not accept any changes

to a route for a specified period of time

Point of using holddown timers- This means that routers will leave a route marked asunreachable in that state for a period of time that is longenough for updates to propagate the routing tables with themost current information.

Routing Loops


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Holddown timers

Holddown timers work in the following way:

A router receives an update from a neighbor indicating thata network that previously was accessible is now no longeraccessible.

Refer to 4.4.4.1


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Routing Loops

The Split Horizon Rule is used to prevent routing loops Split Horizon rule:

A router should not advertise a network throughthe interface from which the update came.


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Routing Loops

Split horizon with poisonreverse: The rule statesthat once a router learnsan unreachable routethrough an interface,

advertise it asunreachable back throughthe same interface


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Routing Loops

IP & TTL Purpose of the TTL field

The TTL field is found in an IP header and is usedto prevent packets from endlessly traveling on anetwork


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Routing Loops

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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Routing Protocols Today

Factors used to determine whether to use RIP or EIGRPinclude

Network size

Compatibility between models of routers

Administrative knowledge


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RIP

Features of RIP:

Supports split horizon & split horizon with poisonreverse

Capable of load balancing Easy to configure

Works in a multi vendor router environment


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EIGRP Features of EIGRP:

Triggered updates.

EIGRP hello protocol used to establish neighboradjacencies.

Supports VLSM & route summarization. Use of topology table to maintain all routes.

Classless distance vector routing protocol.

Cisco proprietary protocol.


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Summary

Characteristics of Distance Vector routing protocols Periodic updates

RIP routing updates include the entire routing table

Neighbors are defined as routers that share a linkand are configured to use the same protocol

The network discovery process for D.V. routingprotocol

Directly connected routes are placed in routingtable 1st

If a routing protocol is configured then Routers will exchange routing information

Convergence is reached when all network routershave the same network information


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Summary

D.V. routing protocols maintains routing tables by RIP sending out periodic updates

RIP using 4 different timers to ensure information isaccurate and convergence is achieved in a timelymanner

EIGRP sending out triggered updates

D.V. routing protocols may be prone to routing loops routing loops are a condition in which packets

continuously traverse a network

Mechanisms used to minimize routing loops includedefining maximum hop count, holddown timers, splithorizon, route poisoning and triggered updates


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Summary

Conditions that can lead to routing loops include Incorrectly configured static routes

Incorrectly configured route redistribution

Slow convergence

Incorrectly configured discard routes

How routing loops can impact network performanceincludes:

Excess use of bandwidth

CPU resources may be strained

Network convergence is degraded

Routing updates may be lost or not processed

S


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Summary

Routing Information Protocol (RIP) A distance vector protocol that has 2 versions

RIPv1 a classful routing protocol

RIPv2 - a classless routing protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) A distance vector routing protocols that has some

features of link state routing protocols

A Cisco proprietary routing protocol


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