Split horizon dictates that a routing update received on an
interface cannot be retransmitted out of the same interface. This rule holds
even if the routing update was received on one Frame Relay PVC and destined for
retransmission onto another Frame Relay PVC. With reference to Figure
, this would mean
that Sites B and C can exchange routing information with Site A, but would not
be able to exchange routing information with each other. Split horizon does not
allow Site A to send routing updates received from Site B on to Site C, and
vice versa.
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NOTE:
For TCP/IP, Cisco routers can disable split horizon on all Frame Relay
interfaces and multipoint subinterfaces and, in fact, do this by default for
most IP routing protocols. However, split horizon cannot be disabled for other
routing protocols, such as those used with IPX and AppleTalk. These other
protocols must use subinterfaces if dynamic routing is desired.
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By dividing the partially meshed Frame Relay network into numerous
virtual, point-to-point networks using subinterfaces, the split-horizon problem
can be overcome.
Each new
point-to-point subnetwork is assigned its own network number. To the routed
protocol, each subnetwork now appears to be located on a separate interface.
Routing updates received from Site B on one logical point-to-point subinterface
can be forwarded to Site C on a separate logical interface without violating
split horizon.