Figure 62 LeHand advertises subnet 192168219226 to Tully as an internal route Network 19216830 is advertised to Tully as a system route and 19216810 is advertised as an external route

However, the local network for LeHand and Thompson is 192.168.3.0. LeHand is the boundary router between major networks 192.168.2.0 and 192.168.3.0, so 192.168.2.0 will be advertised to Thompson as a system route. Likewise, 192.168.3.0 is advertised to Tully as a system route.

192.168.1.0 is a network in another autonomous system, and LeHand has been configured to advertise that network address as a default route.

192.168.1.0 will therefore be advertised to both Thompson and Tully as an external route. IGRP Timers and Stability Features

The IGRP update period is 90 seconds. A random jitter variable of up to 20% is subtracted from each update time to prevent update timer synchronization, so the time elapsed between individual updates will vary from 72 to 90 seconds.

When a route is first learned, the invalid timer for that route is set for 270 seconds, or three times the update period. The flush timer is set for 630 seconds—seven times the update period. Each time an update is received for the route, these timers are reinitialized. If the invalid timer expires before an update is heard, the route is marked as unreachable. It will be held in the routing table and advertised as unreachable until the flush timer expires, at which time the route will be deleted from the table.

The 90-second timer used by IGRP, in comparison to the 30-second timer used by RIP, means that, compared to RIP, IGRP uses less bandwidth for periodic updates. However, the trade-off is that in some cases IGRP may be slower to converge than RIP. For example, if a router goes offline, IGRP takes three times as long as RIP to detect the dead neighbor.

If a destination becomes unreachable or if the next-hop router increases the metric of a destination enough to cause a triggered update, the route will be placed in holddown for 280 seconds (three update periods plus 10 seconds). Until the holddown timer expires, no new information will be accepted about this destination. IGRP holddown may be disabled with the command no metric holddown. In loop-free topologies, where holddown has no real benefit, disabling the function can reduce reconvergence time.

The default timers can be changed with the following command:

timers basic update invalid holddown flush [sleeptime]

This command is also used to manipulate RIP timers with the exception of the sleeptime option. Sleeptime is a timer used to specify a period, in milliseconds, to delay a regular routing update after receiving a triggered update.

The default timers should be changed only in response to an observable problem and only after careful consideration of the consequences. For example, the periods might be reduced to speed up reconvergence in an unstable topology. The price to be paid is increased update traffic—which might contribute to congestion on low-bandwidth links—and an increased number of router CPU cycles to handle the updates. Care must be taken to ensure that the timers are adjusted equally throughout an autonomous system, and configuration management must ensure that any routers added to the autonomous system in the future also are configured with the modified timers.

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