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 maps on the physical interface, as we discussed in Recipe 10.3. In that case, the interface will only change state if the signaling on the physical interface fails. For this reason, we recommend using multipoint configuration instead of frame-relay maps.

As an interesting aside, you can configure frame-relay maps on a multipoint subinterface. This could be useful if you need to mix point-to-point and multipoint interface types on a network that doesn't support inverse ARP.

Note that, as we mentioned in Recipe 10.1, it is not a simple matter to change the subinterface type between point-to-point and multipoint. If you have already defined a subinterface as one type and you want to change it to the other, you must delete the subinterface and reboot the router. Then you can add the subinterface configuration back to the router with the correct type.

The biggest difference between using multipoint subinterfaces and any of the previous examples is that here we do not configure a static mapping between a particular DLCI number and a remote IP address. So, to make this association, the router must use Inverse ARP. If you disable Inverse ARP here, as we did in the previous recipes, the network will not work.

You can see that Inverse ARP is correctly mapping IP addresses to DLCI numbers with the show frame-relay map command:

Central#show frame-relay map
Serial0.1 (up): ip 192.168.55.6 dlci 100(0x64,0x1840), dynamic,
 broadcast,, status defined, active
Central#

If we reconfigure the same DLCI to use a point-to-point subinterface instead (which requires rebooting the router, as we mentioned earlier), you see the output of the show frame-relay map command changes significantly to tell you about the static mapping:

Central#show frame-relay map
Serial0.1 (up): point-to-point dlci, dlci 100(0x64,0x1840), broadcast
 status defined, active
Central#