Frame Relay Verification

show frame-relay pvc lists useful management information, such as the packet counters for each VC, the counters for FECN and BECN.  Comparing packets/bytes sent on one router versus the counters of what is received on the router on the other end of the VC is useful.  This reflects the number of packets/bytes lost inside the Frame Relay cloud.  Also, the PVC status is a great place to start when troubleshooting.

show frame-relay map lists mapping information.  Mappings are only needed when more than two VCs terminate on the interface or subinterface, because those are the only instances in which confusion about which DLCI to use might occur.

debug frame-relay lmi lists information for the sending and receiving LMI inquiries.  The switch sends the status message and the DTE (router) sends the status inquiry.The default setting with Cisco IOS is to send, and to expect to receive, those status messages.  The Cisco IOS no keepalive command is used to disable the use of LMI status messages.  Unlike other interfaces, Cisco keepalive messages do not flow from router to router over Frame Relay.  Instead, they are used to detect whether the router has connectivity to its local Frame Relay switch.

A Partially Meshed Network with Some Fully Meshed Parts

You can also choose to use multipoint subinterfaces for a Frame Relay configuration.  Multipoint subinterfaces work best when you have a full mesh between a set of routers.

The term multipoint simply means that there is more than one VC, so you can send and receive to and from more than one VC on the subinterface.  Like point-to-point subinterfaces, multipoint subinterfaces use the frame-relay interface-dlci command.

Only one frame-relay interface-dlci command is allowed on a point-to-point subinterface, because only one VC is allowed.

No mapping is ever needed for point-to-point subinterfaces, because the only DLCI associated with the interface is statically configured with the frame-relay interface-dlci command.

show frame-relay map shows the mapping information learned by Inverse ARP for multipoint subinterfaces.  The output includes Layer 3 addresses, whereas it wouldn’t for point-to-point subinterfaces.  The reason is that the routes might refer to a next-hop IP address reachable out a multipoint interface, but because more than one DLCI could be associated with the interface, the router needs mapping information to match the next-hop IP address to the correct DLCI.