Split Horizon Rules for Frame Relay

[rossua994]

Can anyone please clarify the split horizon rules for Frame Relay. I have read conflicting descriptions from different sources.

As I understand it, for the main physical interface configured with encapsulation type Frame Relay, split horizon is disabled by default. This is unless EIGRP is being used in which case the command :-

no ip split-horizon eigrp

must be issued explicitly on the interface.

However what happens with sub-interfaces ? What is the default for split horizon for point-to-point subinterfaces, and for multipoint subinterfaces ?

Also, if split horizon is disabled, cannot routing loops then occur, as this is the reason for implementing split horizon in the first place ?

[mohammedmahmoud]

Hi Ross,

IP split horizon checking is disabled by default for Frame Relay encapsulation to allow routing updates to go in and out of the same interface (Physical). An exception is the Enhanced Interior Gateway Routing Protocol (EIGRP) for which split horizon must be explicitly disabled.

Split horizon is not disabled for subinterfaces by default. Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows you to overcome split horizon rules so packets received on one virtual interface can be forwarded to another virtual interface, even if they are configured on the same physical interface.

HTH,

Mohammed Mahmoud.