CCNA2 Module 7

8/8/2019 CCNA2 Module 7

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CCNA2

DYNAMIC ROUTING


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DYNAMIC ROUTING

Dynamic routing protocols can help simplify

the life of a network administrator Routing Information Protocol (RIP) is a

distance vector routing protocol that is usedin thousands of networks throughout the

world. The fact that RIP is based on open standards

and is very simple to implement makes itattractive to some network administrators


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Distance VectorRouting

Routing table updates occur periodically or

when the topology in a distance vectorprotocol network changes

Routing loops can occur when inconsistent

routing tables are not updated due to slow

convergence in a changing network


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Just before the failure of Network 1, all

routers have consistent knowledge andcorrect routing tables. The network is said to

have converged. Assume for the remainder

of this example that Router C's preferred

path to Network 1 is by way of Router B, andthe distance from Router C to Network 1 is 3.


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When Network 1 fails, Router E sends an update to

Router A. Router A stops routing packets to Network1, 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. To Router C, Network1 is still reachable via Router B.


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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 fromRouter C to B to A to D and back to again to C.


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The routing protocol permits the routing loop

to continue until the metric exceeds itsmaximum allowed value

When the metric value exceeds the

maximum value, Network 1 is considered

unreachable.


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Split horizon

Another possible source for a routing loop

occurs when incorrect information that hasbeen sent back to a router contradicts the

correct information that the router originally

distributed.


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Split horizon

Split-horizon attempts to avoid this situation.

If a routing update about Network 1 arrivesfrom Router A, Router B or Router D cannot

send information about Network 1 back to

Router A. Split-horizon thus reduces

incorrect routing information and reducesrouting overhead.


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Split horizon


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Split horizon


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Routepoisoning

Route poisoning is used by various distance

vector protocols in order to overcome largerouting 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.


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Routepoisoning


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Routepoisoning

One way to avoid inconsistent updates is

route poisoning. When Network 5 goesdown, Router E initiates route poisoning by

making a table entry for Network 5 as 16, or

unreachable.

By this poisoning of the route to Network 5,Router C is not susceptible to incorrect

updates about the route to Network 5


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Triggeredupdates

New routing tables are sent to neighboring

routers on a regular basis. For example, RIPupdates occur every 30 seconds

However a triggered update is sent

immediately in response to some change in

the routing table.


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Triggeredupdates

The router that detects a topology change

immediately sends an update message toadjacent routers that, in turn, generate

triggered updates notifying their adjacent

neighbors of the change

When a route fails, an update is sentimmediately rather than waiting on the

update timer to expire


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Holddowntimers

A state where the router cannot advertise or

accept advertisement A router is placed in holdtime when a link of

the router fails


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Holddowntimers

Holddown timers help prevent counting to

infinity but also increase convergence time.The default holddown for RIP is 180 seconds

The holddown timer can be decreased to

speed up convergence


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Holddowntimers

When a router receives an update from a neighbor

indicating that a previously accessible network isnow inaccessible, the router marks the route as

inaccessible and starts a hold down timer

If at any time before the hold down timer expires an

update is received from the same neighbor indicating

that the network is again accessible, the routermarks the network as accessible and removes the

hold down timer.


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Holddowntimers

If an update arrives from a different

neighboring router with a better metric thanoriginally recorded for the network, the routermarks the network as accessible andremoves the holddown timer.

If at any time before the holddown timerexpires an update is received from a differentneighboring router with a poorer metric, theupdate is ignored.


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RIP routingprocess

RIP has evolved over the years from a

Classful Routing Protocol, RIP Version 1(RIP v1), to a Classless Routing Protocol,

RIP Version 2 (RIP v2). RIP v2

enhancements include:

Ability to carry additional packet routing information. Authentication mechanism to secure table updates.

Supports variable length subnet masking (VLSM).


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RIP routingprocess

RIP prevents routing loops from continuingindefinitely by implementing a limit on the number of

hops allowed in a path from the source to adestination

When a router receives a routing update thatcontains a new or changed entry, the metric value isincreased by 1

RIP implements split horizon and holddownmechanisms to prevent incorrect routing informationfrom being propagated.


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RIP routingprocess


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RIP routingprocess


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RIP routingprocess

router running RIP can be configured to send

a triggered update when the networktopology changes using the ipriptriggeredcommand

This command is issued only on serialinterfaces at the router(config-if)#

These updates, called triggered updates, aresent independently of the regularlyscheduled updates that RIP routers forward


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RIP routingprocess

IP Classless command

If a router receives packets for a subnet not in therouting table of the router and the ip classlesscommand is disable the packet will be discarded

classless command is enabled by default inCisco IOSSoftware Release 11.3 and later.

To disable this feature, use the no form of thiscommand.

Is a global configuration command


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RIP routingprocess

Convergence is when all routers in the same

internetwork have the same routinginformation.


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RIP routingprocess

There are several commands that can be

used to verify that RIP is properly configured. Two of the most common are the showip

route command and the showipprotocols

command


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RIP routingprocess

Most of the RIP configuration errors involve

an incorrect network statement,discontiguous subnets, or split horizons. One

highly effective command for finding RIP

update issues is the debugiprip command

The debugiprip command displays RIProuting updates as they are sent and

received


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RIP routingprocess

Using the passiveinterface command can

prevent routers from sending routing updatesthrough a router interface

Keeping routing update messages from

being sent through a router interface

prevents other systems on that network fromlearning about routes dynamically.


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RIP routingprocess

For RIP and IGRP, the passiveinterface

command stops the router from sending

updates to a particular neighbor, but the router

continues to listen and use routing updates

from that neighbor


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RIP routingprocess

Load-balancing describes the ability of a

router to transmit packets to a destination IPaddress over more than one path

The paths are derived either statically or with

dynamic protocols, such as RIP, EIGRP,

OSPF, and IGRP.


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RIP routingprocess


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IGRP

IGRP is a distance vector routing protocol

developed by Cisco. IGRP sends routing updates at 90 second

intervals

By default, the IGRP routing protocol uses

bandwidth and delay as metrics


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IGRP

Additionally, IGRP can be configured to use

a combination of variables to determine acomposite metric. Those variables include:

Bandwidth

Delay

Load Reliability


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IGRP

The composite metric of IGRP is more

accurate than the hop count metric that RIPuses when choosing a path to a destination.

The path that has the smallest metric value

is the best route.


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IGRP

The metrics that IGRP uses are:

Bandwidth The lowest bandwidth value in the path

Delay The cumulative interface delay along the path

Reliability The reliability on the link towards the destinationas determined by the exchange of keepalives

Load The load on a link towards the destination based on bits

per second MTU The Maximum Transmission Unit value of the path.


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IGRP

By default, only bandwidth and delay are

considered. The other parameters areconsidered only if enabled via configuration.

A link with a higher bandwidth will have a

lower metric, and a route with a lower

cumulative delay will have a lower metric.


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IGRP

IGRP advertises three types of routes:

Interior System

Exterior


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IGRP


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IGRP

IGRP has a number of features that are

designed to enhance its stability, such as: Holddowns

Split horizons

Poison reverse updates


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IGRP

To configure the IGRP routing process, use

the routerigrp configuration command. Toshut down an IGRP routing process, use the

no form of this command.

RouterA(config)#routerigrp as-number

RouterA(config)#norouterigrp as-number


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IGRP


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IGRP


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CCNA2 Module 7