System information

Troubleshooting TCP/IP
Book Title
7-108
RIP routing information learned by Router 2 from Router 1 is redistributed into the IGRP domain.
IGRP routing updates are sent to Router 3 from Router 2. If split horizon is disabled on Router 3,
Router 3’s updates to Router 2 will include information about network 193.10.1.0 (which was
originally learned from RIP updates sent from Router 1 to Router 2).
Because IGRP routes by default are given a lower (better) administrative distance than RIP routes,
Router 2 will route traffic to network 193.10.1.0 out serial interface 0 (toward Router 3) rather than
out Ethernet interface 1 (toward Router 1).
Enabling split horizon on Router 3’s serial interface prevents the router from advertising any of the
RIP routes it has learned. However, in some cases, enabling split horizon is not desirable (for
example, in a hub-and-spoke environment). In such a situation, route filtering using an input
distribution list can be configured on Router 2’s serial interface 0, as the following example shows:
Router_2(config)#router igrp 100
Router_2(config-router)#distribute-list 5 in
Router_2(config)#access-list 5 deny 193.10.1.0 255.255.255.0
Router_2(config)#access-list 5 permit 168.170.69.0 255.255.255.0
The syntax for the distribute-list command is as follows:
distribute-list {access-list-number | name} in [type number]
• access-list-number | name—Standard IP access list number or name. The list defines which
networks are to be received and which are to be suppressed in routing updates.
• in—Applies the access list to incoming routing updates.
• type—(Optional) Interface type.
• number—(Optional) Interface number on which the access list should be applied to incoming
updates. If no interface is specified, the access list will be applied to all incoming updates.
This distribution list specifically denies routing updates from Router 3 that advertise network
193.10.1.0, thus preventing Router 2 from learning information about this network from the wrong
protocol and the wrong interface. Be sure to configure explicit permit statements for any traffic that
you do want Router 2 to accept.