8.3 Frame Relay Topologies
8.3.3 Solution for split horizon issues - subinterfaces
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.