Page 326

280

Chapter 5

IP Routing

Holddowns prevent routes from changing too rapidly by allowing time

for either the downed route to come back or the network to stabilize some-
what before changing to the next best route. These also tell routers to restrict,
for a specific time period, any changes that might affect recently removed
routes. This prevents inoperative routes from being prematurely restored to
other routers' tables.

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

viously accessible network is not working and is inaccessible, the holddown
timer will start. If a new update arrives from a neighbor with a better metric
than the original network entry, the holddown is removed and data is
passed. However, if an update is received from a neighbor router before the
holddown timer expires and it has an equal or lower metric than the previous
route, the update is ignored and the holddown timer keeps ticking. This
allows more time for the network to stabilize before trying to converge.

Holddowns use triggered updates, which reset the holddown timer, to

alert the neighbor routers of a change in the network. Unlike update mes-
sages from neighbor routers, triggered updates create a new routing table
that is sent immediately to neighbor routers because a change was detected
in the internetwork.

There are three instances when triggered updates will reset the holddown

timer:

Memorize these!

The holddown timer expires.

Another update is received with a better metric.

A flush timer, which is the time a route would be held before being
removed, removes the route from the routing table.

Routing Information Protocol (RIP)

outing Information Protocol (RIP) is a true distance-vector routing

protocol. It sends the complete routing table out to all active interfaces every
30 seconds. RIP only uses hop count to determine the best way to a remote

www.sybex.com