392 Chapter 11: Frame Relay

Multipoint subinterfaces logically terminate more than one VC. In fact, the name “multipoint” implies the function, because more than one remote site can be reached via a VC associated with a multipoint subinterface.

Table 11-8 summarizes the addresses and subinterfaces that are used in Figure 11-13.

Table 11-8 IP Addresses with Point-to-Point and Multipoint Subinterfaces

What will you see in a real network? Most of the time, point-to-point subinterfaces are used, with a single subnet per PVC.

The later section “Frame Relay Configuration” provides full configurations for all three cases illustrated in Figures 11-11, 11-12, and 11-13.

Broadcast Handling

After contending with Layer 3 addressing over Frame Relay, the next consideration is how to deal with Layer 3 broadcasts. Frame Relay can send copies of a broadcast over all VCs, but there is no equivalent to LAN broadcasts. In other words, no capability exists for a Frame Relay DTE to send a single frame into the Frame Relay network and have that frame replicated and delivered across multiple VCs to multiple destinations. However, routers need to send broadcasts for several features to work. In particular, routing protocol updates are often broadcasts.

The solution to the Frame Relay broadcast dilemma has two parts. First, Cisco IOS software sends copies of the broadcasts across each VC, assuming that you have configured the router to forward these necessary broadcasts. If there are only a few VCs, this is not a big problem. However, if hundreds of VCs terminate in one router, for each broadcast, hundreds of copies could be sent.