Layer 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 either broadcasts or multicasts.

The solution to the Frame Relay broadcast dilemma has two parts.  First, Cisco IOS 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.

The router places the copies of the broadcasts in a different output queue than the one for user traffic so that the user does not experience a large spike in delay each time a broadcast is replicated and sent over every VC.  Cisco IOS can also be configured to limit the amount of bandwidth that is used for these replicated broadcasts.