Cisco IOS Cookbook, 2nd Edition

Using Multipoint Subinterfaces

Problem

You want to configure Frame Relay so that many PVCs share the same subinterface.

Solution

You can connect several virtual circuits to a single subinterface as follows:

Central#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Central(config)#interface Serial0.1 multipoint
Central(config-subif)#description Frame Relay to branches
Central(config-subif)#ip address 192.168.1.1 255.255.255.0
Central(config-subif)#frame-relay interface-dlci 101
Central(config-subif)#frame-relay interface-dlci 102
Central(config-subif)#frame-relay interface-dlci 103
Central(config-subif)#frame-relay interface-dlci 104
Central(config-subif)#exit
Central(config)#end
Central#

Discussion

Recipe 10.1 showed how to create a separate subinterface for each Frame Relay DLCI. Recipe 10.3 showed how to configure all of the DLCIs to share the same interface and the same address range. This recipe shows a method that is somewhere in between these two extremes, with several virtual circuits sharing a common subinterface. You can even combine these multipoint subinterfaces with point-to-point subinterfaces on the same physical interface if you wish to create a hybrid of the two styles.

When you use multipoint configuration as in this recipe, the subinterface will appear to be active unless all of the DLCIs associated with it become unavailable. Then the subinterface will go into a down state and the router will send a trap. This is different from using frame-relay ...