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Chapter 2
Protocol Attributes
IGRP is a classfull protocol, which means it doesn't include any subnet
information about the network with route information. Three types of
routes are recognized by IGRP:
Interior Networks directly connected to a router interface.
System Routes advertised by other IGRP neighbors within the same AS
(autonomous system). The AS number identifies the IGRP session because
it's possible for a router to have multiple IGRP sessions.
Exterior Routes learned via IGRP from a different AS number, which
provide information used by the router to set the gateway of last resort.
The gateway of last resort is the path that a packet takes if a specific route
isn't found on the router.
Figure 2.12 gives an example of the different route types used by IGRP. In
addition to route types, IGRP can store multiple routes to the same destina-
tion by using the topology table.
T A B L E 2 . 7
IGRP Features
Feature
Description
Configurable
metrics
Metrics involved in the algorithm responsible for calcu-
lating route information. They may be configured by
the user.
Flash update
Updates are sent out prior to the default time setting.
This occurs when the metrics for a route change.
Poison reverse
updates
Implemented to prevent routing loops. These updates
place a route in holddown. Holddown means that the
router won't accept any new route information on a giv-
en route for a certain period.
Unequal-cost
load balancing
Allows packets to be shared/distributed across multiple
paths.
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