To enable the split horizon mechanism, use the ip split-horizon interface configuration command. To disable the split horizon mechanism, use the no form of this command.
ip split-horizon
no ip split-horizon
Syntax Description
This command has no arguments or keywords.
Default
Varies with media
Command Mode
Interface configuration
Usage Guidelines
This command first appeared in Cisco IOS Release 10.0.
For all interfaces except those for which either Frame Relay or SMDS encapsulation is enabled, the default condition for this command is ip split-horizon; in other words, the split horizon feature is active. If the interface configuration includes either the encapsulation frame-relay or encapsulation smds commands, then the default is for split horizon to be disabled. Split horizon is not disabled by default for interfaces using any of the X.25 encapsulations.
Note: For networks that include links over X.25 PSNs, the neighbor router configuration command can be used to defeat the split horizon feature. You can as an alternative explicitly specify the no ip split-horizon command in your configuration. However, if you do so you must similarly disable split horizon for all routers in any relevant multicast groups on that network.
If split horizon has been disabled on an interface and you wish to enable it, use the ip split-horizon command to restore the split horizon mechanism.
Note: In general, changing the state of the default for the ip split-horizon command is not recommended, unless you are certain that your application requires a change in order to properly advertise routes. If split horizon is disabled on a serial interface (and that interface is attached to a packet-switched network), you must disable split horizon for all routers and access servers in any relevant multicast groups on that network.
Example
The following simple example disables split horizon on a serial link. The serial link is connected to an X.25 network:
interface serial 0 encapsulation x25 no ip split-horizon