The update interval for IGRP is 90 seconds by default. IGRP uses a random factor of 20 percent
to stagger updates, so the update interval is actually between 72 and 90 seconds.
By default, when a route is first learned, its associated invalid timer is set to three times the
update interval, or 270 seconds, and its associated flush timer is set to seven times the update
interval, or 630 seconds. If for some reason the invalid timer expires, the route will be marked
as unreachable. The route will continue to exist in the routing table and be advertised to
neighbors as unreachable until the flush timer expires. Once the flush timer has expired, the
route will be deleted from the routing table.
When an advertising router marks a route as unreachable, the receiving routers will place the
route in a holddown state. During the period a route is in holddown, the router will ignore all
updates about the route. This prevents routing loops when routes become unreachable. A holddown
timer is set to 280 seconds, or three times the update interval plus 10 seconds.
As noted in the “IGRP Convergence” section in Chapter 1, each time a route is received in
a periodic update, the associated timers are reset for that route. For triggered updates received
off-schedule, the invalid and holddown timers are reset, just as they would be for a periodic
update, but the flush timer is reset at the next periodic update time, whether or not an update
TABLE 4 . 1
Configurable metrics The user can configure metrics involved in the algorithm
responsible for calculating route information.
Triggered update Updates are sent out prior to the update interval timer expiring.
This occurs when the metrics for a route change.
Holddown timer Implemented to prevent routing loops. When inferior updates
are received, IGRP places a route in
means that the router won’t accept any new information on a
given route for a certain period of time, except from the source
of the original route.
Unequal-cost load-balancing Allows packets to be shared or distributed across multiple
Interior Gateway Routing Protocol
is received. From then on, the flush timer continues to increment, as long as no valid updates for
that route are received from that neighbor or until the flush timer expires, and the route is
removed from the routing table.
Timers can be adjusted, and the holddown timer can be turned off. Remember that holddown
timers are used to prevent loops in an internetwork. To adjust the timers, you need to go
into router configuration mode for the IGRP AS to which you want to make these adjustments.
Once there, you can enter the following command:
update invalid holddown flush [sleeptime]
In order to disable the holddown timer, enter the following command in router configuration
no metric holddown
Setting the sleeptime allows you to control when a triggered update will be sent. The triggered
update will not be sent until the sleeptime expires. An important note to remember is not to adjust
any timers without proper reason to do so. Incorrectly adjusting timers can use excess bandwidth
and increase convergence time, just to mention a few of the problems that can occur. If you do adjust
any timers on a router, be sure to adjust the timers accordingly for all the other routers participating
in the same IGRP AS. Failing to do so can cause unexpected results from your internetwork.
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