| The most simple answer to resolving
reachability issues brought on by split horizon might seem to be to
simply turn off split horizon. However, two problems exist with this
solution. First, in most cases, only IP allows you to disable split
horizon; Internetwork Packet Exchange (IPX) and AppleTalk do not.
The only way IPX and AppleTalk can disable split horizon is with
Enhanced Interior Gateway Routing Protocol (EIGRP). Second,
disabling split horizon increases the chances of routing loops in
your network.
To enable the forwarding of broadcast
routing updates in a Frame Relay network, you can configure the
router with logically assigned interfaces called subinterfaces.
Subinterfaces are logical subdivisions of a physical interface. In
split-horizon routing environments, routing updates received on one
subinterface can be sent out on another subinterface. In subinterface
configuration, each VC can be configured as a point-to-point
connection, allowing the subinterface to act similar to a leased
line.
A key reason for using subinterfaces
is to allow distance-vector routing protocols to perform properly in
an environment in which split horizon is activated. Subinterfaces
themselves do not enable the forwarding of broadcasts. Instead, each
destination is perceived to be on its own interface. Therefore, the
router receiving a broadcast on subinterface s0.1, for example, is
capable of reforwarding the same routing information out of
interfaces s0.2 and s0.3.
In the Figure, reconsider the
situation in which Branch A sends a routing update to Central. With
subinterfaces, Central receives the update on its interface s0.1
and is, therefore, able to pass to Branch B and Branch C the same
update, sending it out through interfaces s0.2 and s0.3.
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