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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 31 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 (AS) 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
.
•
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 AS 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 AS has more than one connection to an external network, different routers can choose different
exterior routers as the gateway of last resort.
Figure 31-4 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.
This section briefly describes how to configure IGRP. It includes this information:
•
Default IGRP Configuration, page 31-25
•
Understanding Load Balancing and Traffic Distribution Control, page 31-25
•
Configuring Basic IGRP Parameters, page 31-26
•
Configuring Split Horizon, page 31-28
Router
System
Subnet A
Subnet B
Inter
ior
46649
Exterior
Autonomous system 1
Autonomous
system 2
Router
Router