IP ROUTING By

Anil Kumar Vishwakarma

MCA,MCTS,CCNA

ROUTING BASICS

The term routing is used for taking a packet from one device and

sending it through the network to another device on a different network.

To be able to route packets, a router must know, at a minimum, the

following:

• Destination address

• Neighbor routers from which it can learn about remote

networks

• Possible routes to all remote networks

• The best route to each remote network

• How to maintain and verify routing information

ROUTE TYPES

CONFIGURING STATICIP ROUTING

Static routing has the following benefits: There is no overhead on the router CPU, which means you could

possibly buy a cheaper router than if you were using dynamic routing.

There is no bandwidth usage between routers, which means you could possibly save money on WAN links.

It adds security, because the administrator can choose to allow routing access to certain networks only.

Static routing has the following disadvantages: The administrator must really understand the internetwork and

how each router is connected in order to configure routes correctly.

If a network is added to the internetwork, the administrator has to add a route to it on all routers—by hand.

It’s not feasible in large networks because maintaining it would be a full-time job in itself.

STATIC ROUTING

Static Routes built by using IP route command

DEFAULT ROUTING

Default routes are used to route packets with destinations that do

not match any of the other routes in the routing table.

Routers are typically configured with a default route for

Internet-bound traffic, since it is often impractical and

unnecessary to maintain routes to all networks in the Internet.

A default route is actually a special static route that uses this

format:

ip route 0.0.0.0 0.0.0.0 [next-hop-address]

VERIFYING STATIC ROUTE CONFIGURATION

It is important to verify that they are present in the routing table

and that routing is working as expected.

The command show run is used to view the active configuration

in RAM to verify that the static route was entered correctly.

Router#sh run

The show ip route command is used to make sure that the

static route is present in the routing table.

Router#sh ip route

TROUBLESHOOTING STATIC ROUTE If any problem in connection between networks follow this steps:

1. verifying static routes (by using sh ip route) to make sure the

route found in routing table.

2. If the problem exists, run a traceroute command.

DYNAMIC ROUTING OVERVIEW

Routing protocols

Routed protocol

Autonomous systems

Purpose of a routing protocol and

autonomous systems

Identifying the classes of routing protocols

ROUTING PROTOCOLS

It is the communication used between routers.

It allows one router to share information with other routers

regarding the networks .

The information a router gets from another router, using a

routing protocol, is used to build and maintain a routing table.

Examples of routing protocols are: RIP, IGRP, EIGRP and OSPF

ROUTED PROTOCOL

It is used to direct user traffic.

It provides enough information in its network layer address

to allow a packet to be forwarded from one host to another

based on the addressing scheme.

Examples of routed protocols are: IP and IPX.

AUTONOMOUS SYSTEMS

An autonomous system (AS) is a collection of networks under a

common administration sharing a common routing strategy.

To the outside world, an AS is viewed as a single entity.

The AS may be run by one or more operators while presenting a

consistent view of routing to the external world.

PURPOSE OF A ROUTING PROTOCOL AND

AUTONOMOUS SYSTEMS

The goal of a routing protocol is to build and maintain the routing

table.

Routers use routing protocols to manage information received

from other routers .

Autonomous systems (AS) provide the division of the global

internetwork into smaller and more manageable networks.

Each AS has its own set of rules and policies and an AS number

that will uniquely distinguish it from other autonomous systems

throughout the world.

THE TYPES OF ROUTING PROTOCOLS

Most routing algorithms can be classified into one of two categories:

1. Distance Vector

The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork.

As an example: RIP and IGRP.

2. Link State

The link-state approach, also called shortest path first, recreates the exact topology of the entire internetwork.

As an example: OSPF.

3. Hybrid

Mixed between distance vector and link state

As an example: EIGRP.

DISTANCE VECTOR ROUTING

Distance vector routing updates

Distance vector routing loop issues

Defining a maximum count

Elimination routing loops through split

horizon

Route poisoning

Preventing routing loops with holddown

timers

DISTANCE VECTOR ROUTING UPDATES Routing table updates occur periodically or when the topology in a

distance vector protocol network changes.

Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors.

The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router.

An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing the same remote network, the first thing the router checks is the AD.

If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.

If both advertised routes to the same network have the same AD, then routing protocol metrics will be used to find the best path to the remote network.

The routing tables include information about the total path cost as defined by the metrics.

DISTANCE VECTOR ROUTING LOOP

ISSUES

DISTANCE VECTOR ROUTING LOOP ISSUES Routing loops can occur when inconsistent routing tables are not

updated due to slow convergence in a changing network.

Just before the failure of Network 1, all routers have consistent knowledge and correct routing tables.

When 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 been informed of the failure.

When Router A sends out its update, Routers B and D stop routing to Network 1.

However, Router C has not received an update, so Network 1 is still reachable via Router B.

Now Router C sends a periodic update to Router D, indicating a path to Network 1 by way of Router B.

Router D changes its routing table to reflect this good, but incorrect, information, and propagates the information to Router A.

Router A propagates the information to Routers B and E, and so on.

Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.

DEFINING A MAXIMUM COUNT The invalid updates of Network 1 will continue to loop until some other

process stops the looping. This condition, called count to infinity

DEFINING A MAXIMUM COUNT

ELIMINATION LOOPS BY SPLIT HORIZON Split horizon reduces incorrect routing information and routing overhead

in a distance-vector network by enforcing the rule that routing

information cannot be sent back in the direction from which it was

received.

ROUTE POISONING Route poisoning is used to overcome large routing loops and offer

explicit information when a subnet or network is not accessible.

This is usually accomplished by setting the hop count to one more

than the maximum.

HOLDDOWN TIMERS A holddown prevents regular update messages from reinstating a route that

is going up and down (called flapping).

ROUTING INFORMATION

PROTOCOL Characteristics.

RIP Timers.

Configuring RIP Routing.

Verifying the RIP Routing Tables.

Holding Down RIP Propagations.

RIP CHARACTERISTICS

Vector Distance Protocol.

Open standard protocol.

RIP works well in small networks.

Default AD (administrative distance) is 120.

Hop count is (number of routers) used as Metric for path selection.

Maximum Hop count is 15.

RIP v2 RIP v1

Classless Routing Classful Routing

Support for VLSM No support for VLSM

Support for discontiguous networks No support for discontiguous

networks

Use broadcast or multicast to

deliver routing updates.

Use broadcast to deliver routing

updates.

RIP TIMERS Route update timer: Sets the interval (typically 30 seconds) between

periodic routing updates, in which the router sends a complete copy of its routing table out to all neighbors.

Route invalid timer: Determines the length of time that must elapse (180 seconds) before a router determines that a route has become invalid.

Route flush timer: Sets the time between a route becoming invalid and its removal from the routing table (240 seconds).

Holddown timer: routes will enter into the holddown state when an update packet is received that indicated the route is unreachable.

The default is 180 seconds.

CONFIGURING RIP ROUTING

VERIFYING THE RIP ROUTING

TABLES Two of the most common commands are the show ip route command

and the show ip protocols command.

The show ip protocols command shows which routing protocols are carrying IP traffic on the router.

Another command use to for troubleshooting, debug ip rip. To cancel this opertaon, use this command undebug all.

HOLDING DOWN RIP

PROPAGATIONS

You probably don’t want your RIP network advertised everywhere on

your LAN and WAN.

The easiest way is through the passive-interface command.

This command prevents RIP update broadcasts from being sent out a

defined interface, but that same interface can still receive RIP updates.

Router#config t

Router(config)#router rip

Router(config-router)#network 192.168.10.0

Router(config-router)#passive-interface s0/0

INTERIOR GATEWAY ROUTING

PROTOCOL

Characteristics.

IGRP Timers.

Configuring IGRP routing.

Verifying IGRP routing table.

IGRP CHARACTERISTICS

Vector Distance Protocol.

Cisco-proprietary protocol.

IGRP can works well in large networks.

Default AD (administrative distance) is 100.

Uses an autonomous system number.

Uses bandwidth and delay of the line as metric.

Maximum Hop count is 255.

Classful Routing.

Don’t support VLSM and discontiguous networks.

IGRP TIMERS Update timers These specify how frequently routing-update

messages should be sent. The default is 90 seconds.

Invalid timers These specify how long a router should wait before

declaring a route invalid if it doesn’t receive a specific update about

it. The default is three times the update period (270 sec).

Flush timers The default is seven times the routing update period.

If the update timer is 90 seconds by default, then 7 * 90 = 630

seconds elapse before a route will be flushed from the route table.

Holddown timers These specify the holddown period. The default

is three times the update timer period plus 10 seconds (280 sec).

CONFIGURING IGRP ROUTING

The command used to configure IGRP is the same as the one

used to configure RIP routing with one important difference: you

use an autonomous system (AS) number.

Router#config t

Router(config)#router igrp 10

Router(config-router)#net 192.168.10.0

To Delete routing table built by IGRP, use this command.

Router(config)# no router igrp 10

VERIFYING IGRP ROUTING TABLE To verify that IGRP has been configured properly, enter the show

ip route command and look for IGRP routes signified by an "I".

Additional command

show ip protocols

debug ip igrp events

debug ip igrp transactions

Thank You