The goal at this step in the troubleshooting process is to discover the DLCI of the PVC used to reach a particular neighbor, and then find out if the PVC is working.
To determine the correct PVC, particularly if little or no configuration or documentation is available, you have to start with the failed ping command for the neighboring router’s IP address on the Frame Relay network. From there, you can follow this chain of logic:
Step 3a. Discover the IP address and mask of each Frame Relay interface/subinterface (show interfaces, show ip interface brief) and calculate the connected subnets.
Step 3b. Compare the IP address of the neighbor (from the failed ping command) and pick the local interface/subinterface ...