Configuring Split Horizon - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring RIP
Beginning in privileged EXEC mode, follow these steps to set an interface to advertise a summarized
local IP address and to disable split horizon on the interface:
Command
Step 1 configure terminal
Step 2 interface interface-id
Step 3
ip address ip-address subnet-mask
Step 4 ip summary-address rip ip address ip-network mask Configure the IP address to be summarized and the IP
Step 5 no ip split horizon
Step 6 end
Step 7
show ip interface interface-id
Step 8 copy running-config startup-config
To disable IP summarization, use the no ip summary-address rip router configuration command.
In this example, the major net is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary
address of 10.0.0.0 so that 10.2.0.0 is advertised out interface Gigabit Ethernet port 2, and 10.0.0.0 is
not advertised. In the example, if the interface is still in Layer 2 mode (the default), you must enter a no
switchport interface configuration command before entering the ip address interface configuration
command.
If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with
Note
the ip summary-address rip router configuration command) are advertised.
Switch(config)# router rip
Switch(config-router)# interface gi1/0/2
Switch(config-if)# ip address 10.1.5.1 255.255.255.0
Switch(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
Switch(config-if)# no ip split-horizon
Switch(config-if)# exit
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup
Switch(config-router)# end

Configuring Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally
use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks
information about routes from being advertised by a router on any interface from which that information
originated. This feature can optimize communication among multiple routers, especially when links are
broken.
In general, we do not recommend disabling split horizon unless you are certain that your application
Note
requires it to properly advertise routes.
Catalyst 3750 Switch Software Configuration Guide
34-24
Chapter 34
Purpose
Enter global configuration mode.
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Configure the IP address and IP subnet.
network mask.
Disable split horizon on the interface.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Configuring IP Unicast Routing
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