14 - Routing Protocols & Configuration

7/30/2019 14 - Routing Protocols & Configuration

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14 - Routing Protocols & Configuration

By Muhammad Asghar Khan

Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom


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Agenda

www.asghars.blogspot.com2

Network Routes Connected Routes

Static Routes

Default Routes

Routing Protocols Overview Routing Metrics

Hop Count

Bandwidth

Delay Reliability

Maximum Transmission Unit

Cost

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Agenda


Routing Methods Distance Vector Routing

Link-State Routing

Comparing IP Routing Protocols

RIP-2

Configuring RIP-2

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Network Routes


There are three types of network routes:

Connected Routes

Static Routes

Default Routes

Connected Routes

A router adds routes to its routing table for the subnetsconnected to each of the routers interfaces

For this, the router must have an IP address and maskconfigured on the interface (statically with the ip address

command or dynamically using DHCP) and both interfacestatus codes must be up

The show ip route command can be used to verify theconnected routes

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Network Routes


Static Routes

Static routes are manually defined on a router

Static routes are best suited for small networks, such asLANs, where routes rarely change

You can configure static routes using the ip route

command in global configuration mode

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Network Routes


To remove a static route, you can use the no prefix with the iproute command

The show ip route static command lists only staticallyconfigured IP routes

Default Routes

Default route is a special static route

If the router does not match any routes, the router discardsthe packet

A default route is a route that is considered to match alldestination IP addresses

With a default route, when a packets destination IP addressdoes not match any other routes, the router uses the defaultroute for forwarding the packet

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Network Routes


Default routes work best when router with a single serial linkconnecting it to the rest of the enterprise network

The ip route command with a special IP and mask value, each0.0.0.0 is used to define the default route, which meansmatch all packets

The show ip route command output lists a code of S justlike other static routes, but with an * as well. The * means thatthe route might be used as the default route

You could use static routes, static default routes, on all

routers in an internetwork

However, most enterprises use a dynamic routing protocolto learn all the routes

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Routing Protocols Overview


IP routing protocols have one primary goal: to fill the IProuting table with the current best routes it can find

Several routing protocols exist. The most widely used

routing protocols used are:

Routing Information Protocol (RIP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Open Shortest Path First (OSPF)

Routing protocols use different metrics to evaluate thequality of a route

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Hop Count Hop count is the number of routers that need to be

traversed to get to the destination network

Routing protocols prefer routes with fewer hops

RIP uses the hop count metric to choose network routes Bandwidth

Bandwidth is the throughput of the network route to the

destination

Routing protocols prefer routes with larger bandwidth EIGRP uses the bandwidth metric to pick network routes

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Delay

Delay is the time it takes a data packet to reach the

destination

EIGRP uses the delay metric to pick network routes

Reliability Reliability is the percentage of time the route is available

EIGRP considers the reliability metric to pick network

routes

Maximum Transmission Unit MTU is the size, in bytes, of each data packet

EIGRP considers the MTU metric to pick network routes.

EIGRP prefers routes with higher MTU

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Cost

Cost is calculated based on the bandwidth of a networkroute

Cost is 108/bandwidth

The OSPF protocol considers the cost metric to pick

network routes. OSPF prefers routes with lower cost

Routing protocols also use different methods toexchange information about network routes

Distance Vector Routing

Some routing protocols use the distance to a network toevaluate the quality of a network route

Shorter routes (routes with fewer hops) are consideredbetter than longer routes

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The initial routing information exchange between routers

is called convergence

When routers finish exchanging data about networks and

routes they know, the routers have converged

The convergence process can be fairly long in larger

networks

That is one reason why it is not best practice to enable

distance vector routing protocols on core layer routers

Distance vector routing protocols send updates that

contain the whole routing table, this consumes more

resources

Distance vector routing is best suited for access or distri-

bution layer routers

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Distance vector routing can cause routing loops

Distance vector routing is used by the Routing Information

Protocol (RIP) and Interior Gateway Routing Protocol

(IGRP)

IGRP is no longer supported by Cisco. It has been replaced

by EIGRP (Enhanced IGRP)

Link-State Routing

Link-state routing protocols build their routing tables

independently based on route updates they receive from

their neighbors

Link-state protocols do not merge the routing tables of

neighbor routers

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This initial exchange is called the convergence process

Because link-state protocols do not merge their routing

tables, they converge faster than distance vector protocols

It is best practice to use link-state routing protocols or

hybrid protocols on core layer routers

The Open Shortest Path First (OSPF) protocol uses link

state routing

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Interior and Exterior Routing Protocols

IP routing protocols fall into one of two major categories:

Interior Gateway Protocol (IGP): A routing protocol thatwas designed and intended for use inside a singleautonomous system

Exterior Gateway Protocol (EGP): A routing protocol thatwas designed and intended for use between differentautonomous systems

Each autonomous system can be assigned a number,

called an autonomous system number (ASN) Like public IP addresses, ICANN controls the worldwide

rights to assign ASNs

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Routing protocols that must consider class rules arecalled classful routing protocols; those that do not need

to consider class rules are called classless routing

protocol

Classless routing protocols and classful routingprotocols are identified by the following three criteria

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Routing Information Protocol (RIP-2)


Routers using RIP-2 (Distance Vector Algorithm)advertise a small amount of simple information about

each subnet to their neighbors

Their neighbors in turn advertise the information to

their neighbors, and so on, until all routers havelearned the information

The figure on next slide shows RIP-2 advertising a

subnet number, mask, and metric to its neighbors

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1. Router R2 learns a connected route for subnet 172.16.3.0/24

2. R2 sends a routing update to its neighbors, listing a subnet

(172.16.3.0), mask (/24), and a distance, or metric (1 in this

case)

3. R3 hears the routing update, and adds a route to its routing

table for subnet 172.16.3.0/24, referring to R2 as the next-hop router

4. Around the same time, R1 also hears the routing update sent

directly to R1 by R2. R1 then adds a route to its routing table

for subnet 172.16.3.0/24, referring to R2 as the next-hoprouter

5. R1 and R3 then send a routing update to each other, for

subnet 172.16.3.0/24, metric 2

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Distance vector protocols such as RIP-2 repeat this

process continually on a periodic basis

The previous example shows the basics of one routing

protocol

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Configuring RIP-2


The following steps can be used to configure the RIP-2Move into RIP configuration

mode1

Use RIP Version 2 exclusively

2

Enable RIP on the correct

interfaces, only uses a classful

network number as its one

parameter. 3

RIP configuration does not provide a way to enable

RIP on only some of the interfaces in a single Class

A, B, or C, as it enables it on the basis of subnet

number, to disable the sending of RIP updates on

Fa0/1 using the passive-interface RIP subcommand

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Configuring RIP-2


Three show commands can be used to confirm RIP-2 isworking

show ip interface brief

show ip route [rip]

show ip protocols

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Configuring RIP-2


The best way to understand whether RIP is doing its jobis to use the debug ip rip command

This enables the router to generate log messages each

time the router sends and receives a RIP update

Before using the debug command, it is helpful to look

at the routers CPU utilization with the show process

command

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Configuring RIP-2


If CPU utilization is above 30 to 40 percent, be very cautious

when enabling debug options, as this may drive the CPU tothe point of impacting packet forwarding

In some cases, internetworks use multiple IP routingprotocols. In such cases, a router might learn of multipleroutes to a particular subnet using different routingprotocols

In these cases, the metric does not help the router choosewhich route is best, because each routing protocol uses ametric unique to that routing protocol

The router still needs to choose the best route, so IOS solvesthis problem by assigning a numeric value to each routingprotocol

IOS then chooses the route whose routing protocol has thelower number

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Configuring RIP-2


This number is called the administrative distance (AD)

Table lists the AD values for the most common sources

of routing information

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Documents

14 - Routing Protocols & Configuration