with the exception of periodic updates and failed routes. The metric describing subnet
162.11.10.0, received from Router C, was incremented by 1 before advertising about that
subnet to Router B. This metric value represents hop count, which is used by RIP. (IGRP uses
a different metric, which will be discussed later.) The route to 162.11.10.0 that Router B adds
to its routing table refers to Router A as the next router because Router B learned the route from
Router A; Router B knows nothing about the network topology on the "other side" of Router A.
from a neighboring router if that neighboring router quits sending routing updates. Periodic
routing updates are sent by each router. A routing update timer determines how often the
updates are sent. The timer should be equal on all routers, although the timers can be configured
for different values, causing unpredictable (and bad) results. The absence of routing updates for
a preset number of routing timer intervals results in the removal of the routes previously learned
from the router that has become silent.
routing protocols. Most of the issues with distance vector routing protocols arise when working
with networks with multiple paths because loops are very difficult when there is only one path
possible to get to a subnet. Table 6-5 summarizes these issues and lists the names of the
solutions, which are explained in the upcoming text.
with equal metric
or putting multiple routes to the same subnet in the routing table.
updates passing each other over a
single link
interface only if they were not learned from updates entering that
interface.
advertises all routes out an interface, but those learned from
earlier updates coming in that interface are marked with infinite
distance metrics.
updates passing each other over
alternate paths
advertised with an infinite distance metric.
failed, a router waits a certain period of time before believing any
other routing information about that subnet.
waiting on the update timer to expire when a route has failed.
Used in conjunction with route poisoning, this ensures that all
routers know of failed routes before any holddown timers can
expire.