needs to put a DLCI 100 into the frame so that the switch knows to which virtual circuit it
belongs. Somehow, Router A must get from "I have to send this to Router B" to "Which circuit
do I use?" It looks in the Frame Relay map table and notices a direct mapping of the next hop
192.168.1.2 via DLCI 100 and puts it out on virtual circuit 100.
required quite a bit of manual configuration, a solution to simplify the configuration was
needed. Starting with version 10.3, a method called Inverse ARP was introduced, which
eliminated the need for the manual mapping of addresses. (See Figure 10-18.)
When the router is powered up, it learns from the switch that the PVC is active. The router
learns that DLCI 100 is active. The router sends out a frame with DLCI 100 and puts the IP
address (192.168.1.1) of the interface into the body of the packet. It moves through the Frame
Relay network, and the receiving router can say that Router A is at the other end of the PVC.
Router B puts this into the Frame Relay mapping table. This same process happens from Router
B to Router A.
full-mesh environment, there is a possibility of obtaining the wrong Layer 3 address for a
Layer 2 DLCI.
where the rule of split horizon comes into play. Split horizon can be explained with this caveat:
"Do not tell a joke back to the person who told you the joke." In Figure 10-19, Router A is
advertising Network 200 to Router B. Normally Router B would not advertise this back out the
same interface it learned the route from, due to the split horizon rule. If that were the case,
Router C would never learn about Network 200 because you do not have a connection between
Router A and Router C. One of the ways for Router C to learn that network 200 resides on
Router A is to disable split horizon on Router B's serial interface. If Cisco's IGRP is being used,
there is no need to disable split horizon because IGRP does that automatically. Unfortunately,
with IPX RIP and AppleTalk RTMP, you cannot disable split horizon. DECnet has problems
with split horizon as well.