static, rip & igrp)

© 2002, Cisco Systems, Inc. All rights reserved. 1

Determining IP Routes

Module 5

© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-2© 2002, Cisco Systems, Inc. All rights reserved. 2

Routing Overview

© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-3

To route, a router needs to do the following:• Know the destination address

• Identify the sources it can learn from

• Discover possible routes

• Select the best route

• Maintain and verify routing information

What Is Routing?


• Routers must learn destinations that are not directly connected.

What Is Routing? (Cont.)


Static Route• Uses a route that a

network administrator enters into the router manually

Dynamic Route• Uses a route that a

network routing protocol adjusts automatically for topology or traffic changes

Identifying Static and Dynamic Routes


Static Routes

• Configure unidirectional static routes to and from a stub network to allow communications to occur.


• Defines a path to an IP destination network or subnet or host

Router(config)#ip route network [mask] {address | interface}[distance] [permanent]

Static Route Configuration


Static Route Example

• This is a unidirectional route. You must have a route configured in the opposite direction.


Default Routes

• This route allows the stub network to reach all known networks beyond router A.


Verifying the Static Route Configuration

router#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route Gateway of last resort is 0.0.0.0 to network 0.0.0.0 10.0.0.0/8 is subnetted, 1 subnetsC 10.1.1.0 is directly connected, Serial0S* 0.0.0.0/0 is directly connected, Serial0


• Routing protocols are used between routers to determine paths and maintain routing tables.

• Once the path is determined, a router can route a routed protocol.

What Is a Routing Protocol?


• An autonomous system is a collection of networks under a common administrative domain.

• IGPs operate within an autonomous system.

• EGPs connect different autonomous systems.

Autonomous Systems: Interior or Exterior Routing Protocols


Administrative Distance: Ranking Routes


Classes of Routing Protocols


Classful Routing Overview

• Classful routing protocols do not include the subnet mask with the route advertisement.

• Within the same network, consistency of the subnet masks is assumed.

• Summary routes are exchanged between foreign networks.

• Examples of classful routing protocols:

–RIP Version 1 (RIPv1)

– IGRP


Classless Routing Overview

• Classless routing protocols include the subnet mask with the route advertisement.

• Classless routing protocols support variable-length subnet masking (VLSM).

• Summary routes can be manually controlled within the network.

• Examples of classless routing protocols:

–RIP Version 2 (RIPv2)

–EIGRP

–OSPF

– IS-IS


Routing Protocol Comparison Chart


Using the ip classless Command


Distance Vector Routing


• Routers pass periodic copies of routing table to neighbor routers and accumulate distance vectors.

Distance Vector Routing Protocols


• Routers discover the best path to destinations from each neighbor.

Sources of Information and Discovering Routes


Selecting the Best Route with Metrics


• Updates proceed step-by-step from router to router.

Maintaining Routing Information


• Each node maintains the distance from itself to each possible destination network.

Inconsistent Routing Entries


• Slow convergence produces inconsistent routing.

Inconsistent Routing Entries (Cont.)


• Router C concludes that the best path to network 10.4.0.0 is through router B.



• Router A updates its table to reflect the new but erroneous hop count.



• Hop count for network 10.4.0.0 counts to infinity.

Count to Infinity


• Define a limit on the number of hops to prevent infinite loops.

Defining a Maximum


• Packets for network 10.4.0.0 bounce (loop) between routers B and C.

Routing Loops


• It is never useful to send information about a route back in the direction from which the original information came.

Split Horizon


• Routers advertise the distance of routes that have gone down to infinity.

Route Poisoning


• Poison reverse overrides split horizon.

Poison Reverse


• The router keeps an entry for the network’s possible down state, allowing time for other routers to recompute for this topology change.

Holddown Timers


• The router sends updates when a change in its routing table occurs.

Triggered Updates


Distance Vector Operation


Distance Vector Operation (Cont.)


Summary

• Distance vector-based routing algorithms (also known as Bellman-Ford algorithms) pass periodic copies of a routing table from router to router.

• When the topology in a distance vector protocol internetwork changes, routing table updates must occur. As with the network discovery process, topology change updates proceed step-by-step from router to router.

• When maintaining the routing information, inconsistencies can occur if the internetwork’s slow convergence on a new configuration causes incorrect routing entries.


Summary (Cont.)

• The condition called count to infinity arises when routing table updates continue to increase the metric to a destination that cannot be reached, rather than marking the destination as unreachable.

• A routing loop occurs when two or more routers have incorrect routing information indicating that a valid path to an unreachable destination exists through the other routers.

• A number of techniques are available to eliminate routing loops including: split horizon, route poisoning, poison reverse, holddown timers, and triggered updates.


Enabling RIP


• Maximum is 6 paths (default = 4)

• Hop-count metric selects the path

• Routes update every 30 seconds

RIP Overview


• Router configuration– Select routing protocols.

– Specify networks or interfaces.

IP Routing Configuration Tasks


• Defines an IP routing protocol

Router(config)#router protocol [keyword]

• Mandatory configuration command for each IP routing process

• Identifies the physically connected network that routing updates are forwarded to

Router(config-router)#network network-number

Dynamic Routing Configuration


• Starts the RIP routing process

Router(config)#router rip


• Selects participating attached networks

• Requires a major classful network number

RIP Configuration


RIP Configuration Example


Verifying the RIP Configuration


Displaying the IP Routing Table


debug ip rip Command


Summary

• RIP is a distance vector routing protocol that uses hop count as the metric for route selection and broadcasts routing updates every 30 seconds.

• To enable a dynamic routing protocol, you will select the routing protocol and then assign IP network numbers.

• The router rip command specifies RIP as the routing protocol. The network command identifies a participating attached network.

• The show ip commands display information about routing protocols and the routing table.

• Use the debug ip rip command to display information on RIP routing transactions.


Enabling IGRP


• More scalable than RIP

• Sophisticated metric

• Multiple-path support

Introducing IGRP


•Bandwidth

•Delay

•Reliability

•Loading

•MTU

IGRP Composite Metric


•Maximum 6 paths (default = 4)

•Within metric variance

•Next-hop router closer to destination

IGRP Unequal Multiple Paths


Configuring IGRP


• Selects participating attached networks

Router(config)#router igrp autonomous-system

• Defines IGRP as the IP routing protocol


Configuring IGRP (cont.)

Router(config-router)#traffic-share {balanced | min}

• Controls how load-balanced traffic is distributed

Router(config-router)#variance multiplier

• Controls IGRP load balancing


IGRP Configuration Example


Verifying the IGRP Configuration


Displaying the IP Routing Table


debug ip igrp transaction Command


debug ip igrp events Command

RouterA#debug ip igrp eventsIGRP event debugging is onRouterA#00:23:44: IGRP: sending update to 255.255.255.255 via Ethernet0 (172.16.1.1)00:23:44: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.00:23:44: IGRP: Total routes in update: 200:23:44: IGRP: sending update to 255.255.255.255 via Serial2 (10.1.1.1)00:23:45: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.00:23:45: IGRP: Total routes in update: 100:23:48: IGRP: received update from 10.1.1.2 on Serial200:23:48: IGRP: Update contains 1 interior, 1 system, and 0 exterior routes.00:23:48: IGRP: Total routes in update: 2


Updating Routing Information Example


Updating Routing Information Example (Cont.)


Summary

• IGRP has several key features such as increased scalability, a sophisticated metric, and multiple paths.

• IGRP uses a composite routing metric that can include bandwidth, delay, reliability, loading, and MTU value.

• The IGRP composite routing metric supports multiple paths between source and destination.

• Use the router igrp and network commands to create an IGRP routing process. Use the variance and traffic-share commands to configure IGRP load balancing.

• Use the show ip protocols and show ip route commands to display information about your IGRP configuration.

• Use the debug ip igrp transaction command to display transaction information on IGRP routing transactions and the debug ip igrp events command to display a summary of the IGRP routing information.

static, rip & igrp)

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