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Routing Protocol Basics

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Agenda

Administrative Distances

Distance-vector

Routing Protocols

Routing Loops

RIP IGRP

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Administrative Distances

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Administrative Distances

Administrative distance (AD) is:

AD is used to rate the trustworthiness of routing information

received on a router from a neighbor router. An AD is an integer from 0 to 255, where 0 is the most

trusted and 255 means no traffic will be passed via this route.

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Route Source Default AD

Connected interface 0

Static route 1

EIGRP 90

IGRP 100

OSPF 110

RIP 120

External EIGRP 170

Unknown 255 (this route will never be used )

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Distance-VectorRouting Protocols

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Distance-Vector Routing Protocols

The distance-vector routing algorithm passescomplete routing table contents to neighboringrouters, which then combine the received routingtable entries with their own routing tables tocomplete the routers routing table.

Routing by rumor a router receiving an updatefrom a neighbor router believes the information

about remote networks without actually finding outfor itself.

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Routing Loops

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Routing Loops

Routing loops can occur because every router isntupdated simultaneously, or even close to it.

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How to Stop Routing Loops

Maximum hop count

Split horizon

Route poisoning

Holddowns

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Maximum Hop Count

Routing loop problem counting to infinity

The hop count increases indefinitely each time a packet

passes through a router. Solution: define a maximum hop count.

RIP permits a hop count of up to 15.

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Split Horizon

Reduces incorrect routing information and routingoverhead.

Routing information cannot be sent back in the directionfrom it was received.

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Route Poisoning

When a network goes down, a router initiates routepoisoning by advertising that network as 16, orunreachable (sometimes referred to as infinite).

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Holddowns

Holddown prevent routes from changing too rapidly by allowing timefor either the downed route to come back up or the network tostabilize.

When a router receives an update from a neighbor indicating that a

previously accessible network isnt working and is inaccessible, theholddown timer will start.

If a new update arrives from a neighbor with a better metric thanthe original network entry, the holddown is removed and data ispassed.

If an update is received from a neighbor router before the holddowntimer expires and it has an equal or lower metric than the previousroute, the update is ignored.

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Routing Information Protocol(RIP)

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RIP

Distance-vector routing protocol.

Maximum allowable hop count is 15.

RIP version 1 uses only classful routing

all devices inthe network must use the same subnet mask.

RIP version 2 is classless routing send subnet maskinformation with the route updates.

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RIP Timers

Route update timer Sets the interval between periodic routing updates 30 seconds.

Route invalid timer

The length of time that must elapse before a router determinesthat a route has become invalid 180 seconds.

Holddown timer

The amounts of time during which routing information issuppressed 180 seconds.

Route flush timer

The time between a route becoming invalid and its removal fromthe routing table 240 seconds.

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Interior Gateway RoutingProtocol (IGRP)

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IGRP

Cisco proprietary.

IGRP has a maximum hop count of 255 with a defaultof 100.

IGRP metrics: Bandwidth

Delay

Load

Reliability

IGRP uses bandwidth and delay by default as ametric for determining the best route to aninternetwork.

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IGRP Timers

Update timer Sets the interval between periodic routing updates 90 seconds.

Invalid timer

The length of time that must elapse before a router determinesthat a route has become invalid three times the update period.

Holddown timer

The amounts of time during which routing information issuppressed three times the update period plus 10 seconds.

Flush timer

Indicate how much time time should pass before a route should beflushed from the routing table seven times the routing updateperiod.