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ITT562Routing Protocols and Concepts – Chapter 4

DISTANCE VECTOR ROUTINGPROTOCOLS

DISTANCE VECTOR ROUTING PROTOCOL - CHAPTER 4



CHAPTER 4 AIM

• Identify the characteristics of distance vector routingprotocols.

• Describe the network discovery process of distance

vector 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




Routing Protocol Classification

Interior Gateway Protocol

Distance vectorRouting Protocol

Link State RoutingProtocol

EIGRP

RIPV2

RIPV1OSPF

IGRP

IS-IS

INRODUCTION TO DYNAMIC ROUTING PROTOCOL- CHAPTER 3



DISTANCE VECTOR ROUTING PROTOCOLS

• Dynamic routing protocols help the network administrator overcome the time-consuming and exacting process of

configuring and maintaining static routes.

• Imagine maintaining the static routing configurations of

the 28 routers shown in this scenario.Click on how to manage multiple routing protocols in one environment.


http://localhost/var/www/apps/conversion/tmp/scratch_4/Chapter4_Scenario.ppt

http://localhost/var/www/apps/conversion/tmp/Interactive%20Material/EagleII-ICG_04-multipleprotocols_story_2.pdf



























Discussion

• Discuss why the use of a routing protocol may bemore appropriate than user-defined static routes forthis scenario?

– What are the bandwidth implications of using a distancevector protocol in this internetwork?

– How will full, periodic updates impact the amount ofoverhead added to the network?

– How will use of a dynamic protocol impact security as

compared to static routing?

– Would dynamic routing increase or decrease administrativeburden?

– Redundancy and link failure?







RIP

• Routing Information Protocol (RIP) was originally

specified in RFC 1058. It has the following key

characteristics:

• Hop count is used as the metric for path selection.

• If the hop count for a network is greater than 15, RIP

cannot supply a route to that network.

• Routing updates are broadcast or multicast every 30

seconds, by default.




IGRP

• Interior Gateway Routing Protocol (IGRP) is a

proprietary protocol developed by Cisco.

•Bandwidth, delay, load and reliability are used to createa composite metric.

• Routing updates are broadcast every 90 seconds, by

default.

•IGRP is the predecessor of EIGRP and is now obsolete.






EIGRP

• Enhanced IGRP (EIGRP) is a Cisco proprietary distance

vector routing protocol. EIGRP has these key

characteristics:

• It can perform unequal cost load balancing.

• It uses Diffusing Update Algorithm (DUAL) to calculate

the shortest path.

•There are no periodic updates as with RIP and IGRP.

• Routing updates are sent only when there is a change in

the topology.




DISTANCE VECTOR TECHNOLOGY

• Distance vector means that routes are advertised as

vectors of distance and direction. Distance is defined in

terms of a metric such as hop count and direction is

simply the next-hop router or exit interface.

• A router using a distance vector routing protocol does

not have the knowledge of the entire path to a

destination network. Instead the router knows only:

– The direction or interface in which packets should

be forwarded

– The distance or how far it is to the destinationnetwork




DISTANCE VECTOR ROUTING PROTOCOLSCHARACTERISTICS




Distance Vector Routing Protocols

• Distance vector routing protocols is defines in“distance” and “vector” terms.

• Distance – the most simple ‘distance” measurementis hops

• Vector – is direction

• Distance Vector Routing Protocols allows routers tolearn destination networks in terms of how far awaythey are (distance in hops) and in what direction

(vector)





Operation of Distance Vector Routing Protocols

• Some distance vector routing protocols call for the router to

periodically broadcast the entire routing table to each of its

neighbors. This method is inefficient because the updates not only

consume bandwidth but also consume router CPU resources toprocess the updates.

• Distance vector routing protocols share certain characteristics.

Periodic Updates:

• are sent at regular intervals (30 seconds for RIP and 90 seconds for

IGRP). Even if the topology has not changed in several days,periodic updates continue to be sent to all neighbors.

Question:

How do these functions impact convergence time? scalability? administration time?

How do they impact bandwidth utilization on a network?




• Neighbors are routers that share a link and are

configured to use the same routing protocol. The router

is only aware of the network addresses of its own

interfaces and the remote network addresses it can

reach through its neighbors.

• Broadcast Updates are sent to 255.255.255.255.

Neighboring routers that are configured with the same

routing protocol will process the updates.





Entire Routing Table Updates are sent periodically to allneighbors. Neighbors receiving these updates must process the

entire update to find pertinent information and discard the rest.

Some distance vector routing protocols like EIGRP do not send

periodic routing table updates.




PURPOSE OF A ROUTING ALGORITHM

• At the core of the distance vector protocol is the

algorithm.

• The algorithm is used to calculate the best paths and

then send that information to the neighbors.

• An algorithm is a procedure for accomplishing a certain

task, starting at a given initial state and terminating in a

defined end state.

• Different routing protocols use different algorithms to

install routes in the routing table send updates to

neighbors, and make path determination decisions.




PURPOSE OF A ROUTING ALGORITHM

The algorithm used for the routing protocols defines the

following processes:

1. Mechanism for sending and receiving routing

information.

2. Mechanism for calculating the best paths and installing

routes in the routing table.

3.Mechanism for detecting and reacting to topology

changes.




PURPOSE OF A ROUTING ALGORITHM In the figure, R1 and R2 are configured with a routing

protocol. The algorithm sends and receives updates. Both

R1 and R2 then get new information from the update. In

this case, each router learns about a new network.

The algorithm on each router makes its calculations

independently and updates the routing table with the

new information. When the LAN on R2 goes down, the

algorithm

• constructs a "triggered" update and sends it to R1. R1

then

• removes the network from the routing table.




There are many routing protocols available. Which one to choose?

Criteria used to compare routing protocols includes:

1. Time to convergence

2. Scalability

3. Resource usage

4. Implementation & maintenance





Distance Vector Routing Protocol

1. Time to Convergence

• defines how quickly the routers in the network topology share

routing information and reach a state of consistent knowledge.

• The faster the convergence, the more preferable the protocol.

Routing loops can occur when inconsistent routing tables arenot updated due to slow convergence in a changing network.

2. Scalability

• Scalability defines how large a network can become based on

the routing protocol that is deployed. The larger the networkis, the more scalable the routing protocol needs to be.




Distance Vector Routing Protocol

3. Resource Usage

• Resource usage includes the requirements of a routing

protocol such as memory space, CPU utilization, and link

bandwidth utilization. Higher resource requirements

necessitate more powerful hardware to support the

routing protocol operation in addition to the packet

forwarding processes.

4. Implementation and Maintenance

• describes the level of knowledge that is required for a

network administrator to implement and maintain the

network based on the routing protocol deployed




Distance Vector RoutingProtocol



Network Discovery

• When a router powers up, it knows nothing about the

network topology. It does not even know that there are

devices on the other end of its links. The only information

that a router has is from its own saved configuration file

stored in NVRAM.

• When a router boots successfully, it applies the saved

configuration. If the IP addressing is configured correctly,

then the router will initially discover its own directly

connected networks




Network Discovery (Router Start Up)

• Directly connected networks are initially placed in routing

table.




Network Discovery (Exchange of RoutingInformation)

• If a routing protocol is configured then

-Routers will exchange routing information

• Routing updates received from other routers

-Router checks update for new information

• If there is new information:

– Metric is updated

–

New information is stored in routing table




Network Discovery (Exchange of RoutingInformation)




Routing Table Maintenance

• Many distance vector protocols employ periodic updates

to exchange routing information with their neighbors and

to maintain up-to-date routing information in the routing

table. Examples RIP & IGRP.

• The periodic updates refers to the fact that a router sendsthe complete routing table to its neighbors at a

predefined interval. For RIP, these updates are sent

every 30 seconds as a broadcast (255.255.255.255)

whether or not there has been a topology change




Routing Table Maintenance




RIP uses 4 timers

1. Update timer

2. Invalid timer

3. Flush timer

4. Holddown timer

• Invalid Timer.

–If an update has not been received to refresh an existing route

after 180 seconds (the default), the route is marked as invalid bysetting the metric to 16. The route is retained in the routing table

until the flush timer expires.

Routing Table Maintenance -RIP




Routing Table Maintenance -RIP

• Flush Timer. By default, the flush timer is set for 240

seconds, which is 60 seconds longer than the invalid timer.

When the flush timer expires, the route is removed from the

routing table.

• Holddown Timer. This timer stabilizes routinginformation and helps prevent routing loops. Once a route

is marked as unreachable, it must stay in holddown long

enough for all routers in the topology to learn about theunreachable network. By default, the holddown timer is set

for 180 seconds.




Routing Table Maintenance -EIGRP

• EIGRP does not send periodic updates. Instead, EIGRP

sends bounded updates about a route when a path

changes or the metric for that route changes.

• When a new route becomes available or when a route

needs to be removed, EIGRP sends an update only tothose routers that need it.

– EIRPG routing updates are

– Partial updates

– Triggered by topology changes – Bounded

– Non periodic

• Note: More details on how EIGRP operates will be presented in Chapter 9.




Routing Table Maintenance -EIGRP

• How long will a distance vector based internetwork take to converge as

compared to a statically routed network?

• As compared to a statically routed internetwork, how long will a dynamically

routed internetwork take to re-convergence?

• Is a statically routed internetwork more or less scalable than a dynamically

routed internetwork?• Would a router running dynamic routing protocols require more or less CPU

power than a router using static routes?

• What would the impact be on available internetwork network bandwidth?




Routing Loops

• Definitions &Implications

• Routing loops are

– A condition in which a

packet is continuously

transmitted within a

series of routers

without ever reaching

its destination.




Routing Loops

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 destination network.

• A routing loop can occur when two or more routers have

routing information that incorrectly indicates that a valid

path to an unreachable destination exists.




• 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 a timely

manner

Routing Loops




Routing Loops - Count to Infinity

• Infinity is defined by setting a maximum metricvalue.

• For example, RIP defines infinity as 16 hops – an

"unreachable" metric. Once the routers "count to infinity,"

they mark the route as unreachable.




Static RouteRouting Loops - Holddown Timers

• Holddown timers allow a router to not accept any

changes to a route for a specified period of time

• Holddown timers are used to prevent regular update

messages from inappropriately reinstating a route that

may have gone bad. Holddown timers instruct routers to

hold any changes that might affect routes for a specified

period of time.




Routing Loops - Split Horizon Rule

• Split Horizon rule: A router should no advertise a network

through the interface from which the update came.

R i L S li h i i h P i



• Route poisoning is used to

mark the route as

unreachable in a routing

update that is sent to other

routers.• Unreachable is interpreted

as a metric that is set to the

maximum.

• For RIP, a poisoned routehas a metric of 16.

Routing Loops -Split horizon with PoisonReverse or Route Poisoning




Routing Loops

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 a network

How the TTL field works

• TTL field contains a numeric value

• The numeric value is decreased by one by every router

on the route to the destination.• If numeric value reaches 0 then Packet is discarded




Routing Protocols Today

Factors used to determine whether to use RIP orEIGRP

include:

– Network size

– Compatibility between models of routers

– Administrative knowledge





RIP

Features of RIP:

-Supports split horizon & split horizon with poison reverse-Capable of load balancing

-Easy to configure

-Works in a multi vendor router environment





EIGRP

• Features of EIGRP:

– 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

chapter 4 distance vector protocols

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