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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 22 Configuring IP Unicast Routing
Configuring IGRP
Configuring IGRP
Interior Gateway Routing Protocol (IGRP) is a dynamic, distance-vector routing, proprietary Cisco
protocol for routing in an autonomous system that contains large, arbitrarily complex networks with
diverse bandwidth and delay characteristics. IGRP uses a combination of user-configurable metrics,
including internetwork delay, bandwidth, reliability, and load. IGRP also advertises types of routes:
interior, system, and exterior, as shown in
Figure 22-4
.
•
Interior routes are routes between subnets in the network attached to a router interface. If the
network attached to a router is not subnetted, IGRP does not advertise interior routes.
•
System routes are routes to networks within an autonomous system. The router derives system routes
from directly connected network interfaces and system route information provided by other
IGRP-speaking routers or access servers. System routes do not include subnet information.
•
Exterior routes are routes to networks outside the autonomous system that are considered when
identifying a gateway of last resort. The router chooses a gateway of last resort from the list of
exterior routes that IGRP provides if it does not have a better route for a packet and the destination
is not a connected network. If the autonomous system has more than one connection to an external
network, different routers can choose different exterior routers as the gateway of last resort.
Figure 22-4 Interior, System, and Exterior Routes
By default, a router running IGRP sends an update broadcast every 90 seconds and declares a route
inaccessible if it does not receive an update from the first router in the route within three update periods
(270 seconds). After seven update periods (630 seconds), the route is removed from the routing table.
Table 22-6
shows the default IGRP configuration.
Router
System
Subnet A
Subnet B
Inter
ior
46649
Exterior
Autonomous system 1
Autonomous
system 2
Router
Router
Table 22-6 Default IGRP Configuration
Feature
Default Setting
IP split horizon
Varies with media.
Metric holddown
Disabled.
Metric maximum-hops
100 hops.
Neighbor
None defined.
