Configuring Summary Aggregate Addresses; Disabling Eigrp Split Horizon - Cisco PIX 500 Series Configuration Manual

Chapter 9 Configuring IP Routing
For example, if you have a router with the networks 192.168.1.0, 192.168.2.0, and 192.168.3.0
connected to it, and those networks all participate in EIGRP, the EIGRP routing process creates the
summary address 192.168.0.0 for those routes. If an additional router is added to the network with the
networks 192.168.10.0 and 192.168.11.0, and those networks participate in EIGRP, they will also be
summarized as 192.168.0.0. To prevent the possibility of traffic being routed to the wrong location, you
should disable automatic route summarization on the routers creating the conflicting summary
addresses.
To disable automatic router summarization, enter the following command in router configuration mode
for the EIGRP routing process:
hostname(config-router)# no auto-summary
Automatic summary addresses have an adminstrative distance of 5. You cannot configure this value.
Note

Configuring Summary Aggregate Addresses

You can configure a summary addresses on a per-interface basis. You need to manually define summary
addresses if you want to create summary addresses that do not occur at a network number boundary or
if you want to use summary addresses on a security appliance with automatic route summarization
disabled. If any more specific routes are in the routing table, EIGRP will advertise the summary address
out the interface with a metric equal to the minimum of all more specific routes.
To create a summary address, perform the following steps:
Enter interface configuration mode for the interface on which you are creating a summary address by
Step 1
entering the following command:
hostname(config)# interface phy_if
Create the summary address by entering the following command:
Step 2
hostname(config-if)# summary-address eigrp as-num address mask [ distance ]
By default, EIGRP summary addresses that you define have an administrative distance of 5. You can
change this value by specifying the optional distance argument in the summary-address command.

Disabling EIGRP Split Horizon

Split horizon controls the sending of EIGRP update and query packets. When split horizon is enabled on
an interface, update and query packets are not sent for destinations for which this interface is the next
hop. Controlling update and query packets in this manner reduces the possibility of routing loops.
By default, split horizon is enabled on all interfaces.
Split horizon blocks route information from being advertised by a router out of any interface from which
that information originated. This behavior usually optimizes communications among multiple routing
devices, particularly when links are broken. However, with nonbroadcast networks, there may be
situations where this behavior is not desired. For these situations, including networks in which you have
EIGRP configured, you may want to disable split horizon.
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Cisco Security Appliance Command Line Configuration Guide
Configuring EIGRP
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