IGRPIGRP

► II GRP is a distance vector routing protoc GRP is a distance vector routing protoc ol developed by Cisco. ol developed by Cisco.

► IGRP sends routing updates at IGRP sends routing updates at 9090 secon secon d intervals. d intervals.

► These updates advertise all the networ These updates advertise all the networ ks for a particular AS. ks for a particular AS.

Key design characteristics of IGR Key design characteristics of IGRPP

► The versatility to automatically handle i The versatility to automatically handle i ndefinite, complex topologies ndefinite, complex topologies

► The flexibility needed to segment with The flexibility needed to segment with different bandwidth and delay characte different bandwidth and delay characte

ristics ristics► Scalability for functioning in very large Scalability for functioning in very large

networks networks

MetricsMetrics

► By default, the IGRP routing protocol uses bandwidth By default, the IGRP routing protocol uses bandwidth and delay as metrics. and delay as metrics.

► Additionally, IGRP can be configured to use a combin Additionally, IGRP can be configured to use a combin ation of variables to determine a composite metric. ation of variables to determine a composite metric.

► These variables are as follows: These variables are as follows: Bandwidth Bandwidth Delay Delay Load Load Reliability Reliability

► K1 to K5 factors are used by the algorithm to calculat K1 to K5 factors are used by the algorithm to calculat e the routing metric for IGRP. e the routing metric for IGRP.

► By default the values of the factors K1 and K3 are set By default the values of the factors K1 and K3 are set to 1, and K2, K4, and K5 are set to 0. to 1, and K2, K4, and K5 are set to 0.

► The path that has the smallest metric value is the bes The path that has the smallest metric value is the bes t route. t route.

Metric calculationMetric calculation

► The path that has the smallest metric value i The path that has the smallest metric value i s the best route. s the best route.

Metrics detailsMetrics details

► Delay is in units of Delay is in units of 1010 microseconds. This gives a ran microseconds. This gives a ran ge of ge of 1010 microseconds to microseconds to 168168 seconds, which seems seconds, which seems

sufficient. sufficient.► Bandwidth is inverse bandwidth in bits per sec scale Bandwidth is inverse bandwidth in bits per sec scale

d by a factor of d by a factor of 1.01.0ee1010 . The range is from a . The range is from a 12001200 BPS BPS line to line to 1010 Gbps. (That is, if the bandwidth is N Kbps, t Gbps. (That is, if the bandwidth is N Kbps, t

he number used is he number used is 1000000010000000 / N.) / N.)► MTU is in bytes. MTU is in bytes.► Reliability is given as a fraction of Reliability is given as a fraction of 255255 . That is, . That is, 255255 is is

100%100%..► Load is given as a fraction of Load is given as a fraction of 255255..► Hop count is a simple count. Hop count is a simple count.

IGRP route (1/2)IGRP route (1/2)

► Interior routes Interior routes are routes between subnets of are routes between subnets of a network attached to a router interface. a network attached to a router interface.

If the network attached to a router is not subnetted If the network attached to a router is not subnetted , IGRP does not advertise interior routes. , IGRP does not advertise interior routes.

► SS ystem routes ystem routes are routes to networks within a are routes to networks within a n autonomous system. n autonomous system.

The Cisco IOS software derives system routes from The Cisco IOS software derives system routes from directly connected network interfaces and system directly connected network interfaces and system

route information provided by other IGRP routers o route information provided by other IGRP routers o r access servers. r access servers.

System routes do not include subnet information. System routes do not include subnet information.

IGRP route (2/2)IGRP route (2/2)

► Exterior routes Exterior routes are routes to networks outside are routes to networks outside the autonomous system that are considered w the autonomous system that are considered w hen a gateway of last resort is identified. hen a gateway of last resort is identified.

The Cisco IOS software chooses a gateway of last re The Cisco IOS software chooses a gateway of last re sort from the list of exterior routes that IGRP provide sort from the list of exterior routes that IGRP provide

s. s. The software uses the gateway of last resort if a bet The software uses the gateway of last resort if a bet

ter route is not found and the destination is not a co ter route is not found and the destination is not a co nnected network. nnected network.

If the autonomous system has more than one conne If the autonomous system has more than one conne ction to an external network, different routers can c ction to an external network, different routers can c

hoose different exterior routers as the gateway of la hoose different exterior routers as the gateway of la st resort. st resort.

IGRP stability featuresIGRP stability features► HolddownsHolddowns are used to prevent regular updat HolddownsHolddowns are used to prevent regular updat

e messages from reinstating a route that may not be up. e messages from reinstating a route that may not be up. When a router goes down, neighbor routers detect this from the l When a router goes down, neighbor routers detect this from the l

ack of regularly scheduled update messages. ack of regularly scheduled update messages.

► Split horizonsSplit horizons are derived from the premise Split horizonsSplit horizons are derived from the premise that it is not useful to send information about a route bac that it is not useful to send information about a route bac

k in the direction from which it came. k in the direction from which it came. The split horizon rule helps prevent routing loops between adjac The split horizon rule helps prevent routing loops between adjac

ent routers. ent routers.

► Poison reverse updatesPoison reverse updates are used Poison reverse updatesPoison reverse updates are used to prevent larger routing loops. to prevent larger routing loops.

Increases in routing metrics usually indicate routing loops. Increases in routing metrics usually indicate routing loops. Poison reverse updates then are sent to remove the route and pl Poison reverse updates then are sent to remove the route and pl

ace it in holddown. ace it in holddown. With IGRP, poison reverse updates are sent only if a route metric With IGRP, poison reverse updates are sent only if a route metric

has increased by a factor of has increased by a factor of 1.11.1 or greater. or greater.

IGRP TimersIGRP Timers

► The update timer The update timer specifies how frequently routing upd specifies how frequently routing upd ate messages should be sent. ate messages should be sent.

The IGRP default for this variable is The IGRP default for this variable is 9090 seconds. seconds.► The invalid timer The invalid timer specifies how long a router should wa specifies how long a router should wa

- it in the absence of routing update messages about a r - it in the absence of routing update messages about a r oute before it declares that route invalid. oute before it declares that route invalid.

The IGRP default for this variable is three times the update per The IGRP default for this variable is three times the update period.iod.

► The holddown timer The holddown timer specifies the amount of time for w specifies the amount of time for w hich information about poorer routes is ignored. hich information about poorer routes is ignored.

The IGRP default for this variable is three times the update tim The IGRP default for this variable is three times the update tim er period plus er period plus 1010 seconds. seconds.

► TT he flush timer he flush timer indicates how much time should pass b indicates how much time should pass b efore a route is flushed from the routing table. efore a route is flushed from the routing table.

The IGRP default is seven times the routing update timer. The IGRP default is seven times the routing update timer.