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Distance Vector

Routing Protocols

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Distance Vector Routing Updates

• Routing table updates occur periodically or when the topology

in a distance vector protocol network changes.

• Topology change updates proceed systematically from router to

router.

• Distance vector algorithms callfor each router to send its entire

routing table to each of its

adjacent neighbors.

• The routing tables includeinformation about the total path

cost as defined by the metrics.

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Distance Vector Routing Metric

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Convergence

• onvergence is the speed and ability of a group of

internetworking devices running a specific routing protocol to

agree on the topology of an internetwork after a change in that

topology.

• ! network has converged when all routers have consistentknowledge and correct routing tables.

• Routing loops can occur when inconsistent routing tables are

not updated due to slow convergence in a changing network.

• "f a network has not converged# the following are possible$ % "nconsistent routing tables

% "nconsistent traffic forwarding

% Routing loops

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Routing Loops

• "nvalid updates will continue to loop until some process stops

the looping.

• This condition is called count to infinity.

• Packets loop continuously around the network in spite of the

fundamental fact that the destination network is down.

• These packets loop through the network because of wrong

information in the routing tables.

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Routing Loops

• To reduce routing loops and counting to infinity# R"P uses the

following techni&ues$

% ount'to'infinity

% (plit hori)on

% Poison reverse

% *olddown counters

% Triggered updates

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Routing Loops – Count to Infinity

• Distance vector routing algorithms are self'correcting# but a

routing loop problem can re&uire a count to infinity.

• +hen a routing table update is received by a router in a

distance vector network# the hop count for the updated route is

incremented by one.• +ithout countermeasures to stop the count to infinity process#

the distance vector metric of hop count increments each time

the packet passes through another router.

• To avoid this prolonged problem# distance vector protocolsdefine infinity as a specific ma,imum number.

• +ith this approach# the routing protocol permits the routing loop

to continue until the metric e,ceeds its ma,imum allowed

value.

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Routing Loops – Split-Horizon

• !nother possible source for a routing loop occurs when

incorrect information that has been sent back to a router

contradicts the correct information that the router originally

distributed.

• (plit'hori)on attempts to avoid this situation.• "f a routing update about a network arrives from a router# the

router that receives the update cannot send information about

that network back to the router that originally sent the update.

• (plit'hori)on thus reduces incorrect routing information andreduces routing overhead.

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Routing Loops – Split-Horizon

• The split hori)on rule is based on the theory that it is not useful

to send information about a route back in the direction from

which it came.

"f router !

receives an

update from

router D# it would

not send that

information back

to router D.

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Routing Loops – Route oisoning

• Route poisoning is used by various distance vector protocols in

order to overcome large routing loops and offer e,plicit

information when a subnet or network is not accessible.

• Route poisoning accomplishes this by setting the hop count to

one more than the ma,imum.

The ma,imum hop

count for R"P is -.

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Routing !a"le Updates

• /ew routing tables are sent to neighboring routers on a regular

basis 0R"P updates occur every 12 seconds3.

• *owever a triggered update is sent immediately in response to

some change in the routing table.

• The router that detects a topology change immediately sendsan update message to adjacent routers that# in turn# generate

triggered updates notifying their adjacent neighbors of the

change.

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Holddo#n !i$ers

• ! count to infinity problem can be avoided by using holddown

timers

• +hen a router receives an update from a neighbor indicating

that a previously accessible network is now inaccessible# the

router marks the route as inaccessible and starts a holddowntimer

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Holddo#n !i$ers

• "f at any time before the holddown timer e,pires an update is

received from the same neighbor indicating that the network is

again accessible# the router marks the network as accessible

and removes the holddown timer.

• "f an update arrives from a different neighboring router with abetter metric than originally recorded for the network# the router

marks the network as accessible and removes the holddown

timer.

• "f at any time before the holddown timer e,pires an update is

received from a different neighboring router with a poorer

metric# the update is ignored.

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Holddo#n !i$ers

• *olddown timers help prevent counting to infinity but also

increase convergence time.

• The default holddown for R"P is -42 seconds.

• This will prevent any inferior route from being updated but may

also prevent a valid alternative route from being installed.

• The holddown timer can be decreased to speed up

convergence but should be done with caution.

• The ideal setting would be to set the timer just longer that the

longest possible update time for the internetwork.

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Holddo#n !i$ers

• The ideal setting would be to set the timer just longer that the

longest possible update time for the internetwork.

Set t%e %olddo#n ti$er for &ust over

'() second – e*a$ple '(+

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Holddo#n !i$ers

• 5ne additional item that affects convergence time# and is

configurable# is the update interval.

• The default R"P update interval in isco "5( is 12 seconds.

• This can be configured for longer intervals to conserve

bandwidth# or for shorter intervals to decrease convergencetime.

• To change the update internal$

– GAD(config-router)#update-timer seconds

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Routing Infor$ation rotocol ,RI

• R"P is a distance vector routing protocol

• *op count is used as the metric for path selection

• "f the hop count is greater than -# the packet will be discarded

• 6y default# routing updates are broadcast every 12 seconds

• R"P has evolved over the years from a lassful Routing

Protocol# R"P Version - 0R"P v-3# to a lassless Routing

Protocol# R"P Version 7 0R"P v73.

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• R"P prevents routing loops from continuing indefinitely by

implementing a limit on the number of hops allowed in a path

from the source to a destination.

• The ma,imum number of hops in a path is -.

• +hen a router receives a routing update that contains a new orchanged entry# the metric value is increased by - to account for

itself as a hop in the path.

• "f this causes the metric to be incremented beyond -# it is

considered to be infinity and the network destination isconsidered unreachable.

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• R"P sends routing'update messages at regular intervals.

• +hen a router receives a routing update that includes changes

to an entry# it updates its routing table to reflect the new route.

• The received metric value for the path is increased by -# and

the source interface of the update is indicated as the ne,t hopin the routing table.

• R"P routers maintain only the best route to a destination but can

maintain multiple e&ual'cost paths to the destination.

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Configuring RI

• The router rip command enables R"P as the routing

protocol.

• The network command is then used to tell the router on which

interfaces to run R"P.

• The routing process then associates specific interfaces with thenetwork addresses and begins sending and receiving R"P

updates on these interfaces.

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Configuring RI

• Router(config)#router rip

% 8nables the R"P routing process

• Router(config-router)#network network-number

% !ssociates a network with the R"P routing process

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Configuring RI

Ho# #ould a user configure RI on t%e ./D router0

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ip classless Co$$and

• "n order for the isco "5( software to forward packets to the

best supernet route possible# use the ip classless global

configuration command.

• "f the router receives a packet for a subnet that does not have a

default route and ip classless is enabled# it will forward thepacket to the subnet via a supernet route.

• ! supernet route is a route that covers a greater range of

subnets with a single entry.

• 9or e,ample# an enterprise uses the entire subnet -2.-2.2.2 :

-;# then a supernet route for -2.-2.-2.2 :7< would be -2.-2.2.2

:-;.

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ip classless Co$$and

• The ip classless command is enabled by default in isco

"5( (oftware Release --.1 and later.

• To disable this feature# use the no form of this command.

% no ip classless

• +hen this feature is disabled any packets received that are

destined for a subnet that numerically falls within the router=s

subnetwork addressing scheme will be discarded.

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passive-interface co$$and

• !nother issue with routing protocols is the unwanted

advertisement of routing updates out a particular interface.

• +hen a network command is issued for a given network# R"P

will immediately begin sending advertisements out all interfaces

within the specified network address range.• To control the set of interfaces that will e,change routing

updates# the network administrator can disable the sending of

routing updates on specified interfaces by configuring the

passive-interface command.

– GAD(config-router)# passive-interface e0

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Verifying RI

• There are several commands that can be used to verify that

R"P is properly configured.

• Two of the most common methods are$

– show ip route command

– show ip protocols command.

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show ip protocols

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show ip route

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!rou"les%ooting RI

• 5ne highly effective command for finding R"P update issues is

the debug ip rip command.

• The debug ip rip command displays R"P routing updates as

they are sent and received.

(hows activity as

it occurs

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!rou"les%ooting RI

• 5ther commands to troubleshoot R"P$

– show ip rip database

– show ip protocols summar!"

– show ip route

– debug ip rip events" – show ip interface brief

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Load 1alancing #it% RI

• >oad balancing is a concept that allows a router to take

advantage of multiple best paths to a given destination.

• R"P is capable of load balancing over as many as si, e&ual'cost

paths# with four paths being default.

• R"P performs what is referred to as ?round robin@ loadbalancing. This means that R"P takes turns forwarding packets

over the parallel paths.

% The router will start with an interface pointer to the interface

connected to router -.

% Then the interface pointer cycles through the interfaces and routes

in a deterministic fashion such as -'7'1'

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• 1ecause t%e $etric for RI is %op count2 no regard is given

to t%e speed of t%e lin3s4

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• 1ecause t%e $etric for RI is %op count2 no regard is given

to t%e speed of t%e lin3s4

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Load 1alancing

• >oad balancing is a concept that allows a router to take

advantage of multiple best paths to a given destination.

• The paths are derived either statically or with dynamic

protocols# such as R"P# 8"ARP# 5(P9# and "ARP.

• +hen a router learns multiple routes to a specific network# theroute with the lowest administrative distance is installed in the

routing table.

• (ometimes the router must select a route from among many#

learned via the same routing process with the same

administrative distance.

• The router chooses the path with the lowest cost or metric to

the destination.

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Load 1alancing

• 8ach routing process calculates its cost differently and the

costs may need to be manually configured in order to achieve

load balancing.

• R"P networks must have the same hop count to load balance#

whereas "ARP uses bandwidth to determine how to loadbalance.

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/d$inistrative Distance

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Load 1alancing

• +hen routing "P# the isco "5( offers two methods of load

balancing# per'packet and per'destination load balancing.

• "f process switching is enabled# the router will alternate paths

on a per'packet basis.

• "f fast switching is enabled# only one of the alternate routes willbe cached for the destination address# so all packets in the

packet stream bound for a specific host will take the same path.

• Packets bound for a different host on the same network may

use an alternate route# traffic is load balanced on a per'

destination basis.

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I.R

• "ARP is a distance vector "nterior Aateway Protocol 0"AP3.

• isco created this routing protocol to overcome the problems

associated with R"P.

• "ARP converges faster than R"P

• Routers using distance vector protocols must send all or aportion of their routing table in a routing update message at

regular intervals to each of their neighboring routers.

• !s routing information spreads throughout the network# routers

perform the following functions$ % "dentify new destinations

% >earn of failures

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I.R

• "ARP is a distance vector routing protocol developed by isco

0it is a isco proprietary routing protocol3.

• I.R sends routing updates at 5) second intervals#

advertising networks for a particular autonomous system.

• 6y default# the "ARP routing protocol uses bandwidth and delayas metrics.

• !dditionally# "ARP can be configured to use a combination of

variables to determine a composite metric.

% 6andwidth

% Delay

% >oad

% Reliability

! composite metric is more accurate than the hopcount metric that R"P uses when choosing a path

to a destination.

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I.R

• The show ip

protocols

command displays

parameters# filters#

and networkinformation

concerning the

routing protocols in

use on the router.

Default is '))2 $a*4 is (66

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I.R

B• Aiven the following information from the show ip protocols

command# when would the ne,t update interval be e,pectedB

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I.R

"ARP has a set of timers to enhance its performance and

functionality$

• Cpdate Timer$ These specify how fre&uently routing'update

messages should be sent. The default is 2 seconds.

• "nvalid Timers$ These specify how long a router should waitbefore declaring a route invalid if it does not receive a specific

update about it. The default is three times the update period.

• *old'down Timers$ The holddown timer specifies the amount of

time for which information about poorer routes is ignored.The

default is three times the update timer period plus -2 seconds.

• Route 9lush Timer$These indicate how much time should pass

before a route should be flushed from the routing table. The

default is seven times the routing period.

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I.R"f default settings are used# by looking

at the update interval you can tell whatrouting protocol is being used.

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I.R

• The show ip

route command

shows the metric

values in brackets.

• The ?@ verifies that

"ARP is configured.

!dministrative distance : omposite Eetric

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I.R

• "ARP has a number of features that are designed to enhance

its stability# such as$

% *olddowns ' used to prevent regular update messages from

inappropriately reinstating a route that may not be up.

% (plit hori)ons ' derived from the premise that it is usually notuseful to send information about a route back in the direction from

which it came.

% Poison reverse updates % used to defeat larger routing loops#

increases the hop count to one more than the ma,imum hop

count.

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Configuring I.R

• To configure the "ARP routing process# use the router igrp

configuration command. To shut down an "ARP routing

process# use the no form of this command.

– Router(config)#router igrp as-number

• The !utonomous (ystem number is one that identifies the

"ARP process. "t is also used to tag the routing information.

• To specify a list of networks for "ARP routing processes# use

the network router configuration command. To remove a

network# use the no form of this command.

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Configuring I.R

RouterA(config)#router igrp $00

RouterA(config-router)#network $%&'$'$'0

RouterA(config-router)#network $%&'$'&'0

• +hat commands would be needed to configure Router6 for

"ARP with the autonomous system number -22B

192.168.1.32/27

192.168.1.64/27

192.168.2.32/24

192.168.3.32/24

RouterA RouterB

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I.R

• "ARP is showing its age# it lacks support for variable length

subnet masks 0V>(E3.

• Rather than develop an "ARP version 7 to correct this problem#

isco has built upon "ARPFs legacy of success with 8nhanced

"ARP.

Documents

day 10 RIP (2).ppt