Monday’s sample question focused on the data link layer addresses used as frames pass over a Frame Relay WAN. Today, I’ll summarize the key points that must be considered when answering that question, as well as posing a follow-on general question about Frame Relay addressing. (If you’ve not looked at Monday’s question yet, it may be worth reading the question, and picking some answers, before reading these suggestions.)
First, here are a few general concepts that relate in some way to answering Monday’s question:
1. When routers receive and forward a packet, the router discards the incoming frame’s data link header/trailer, and then encapsulates the packet inside a new data link header/trailer appropriate for the outgoing interface.
2. The Frame Relay header holds a single Data Link address, the Data Link Connection Identifier (DLCI). There is no source DLCI and destination DLCI – only a single DLCI field.
3. The DLCI identifies the PVC as the frame passes over the local access link – a feature called local addressing.
4. The router on the other end of a PVC uses a DLCI to identify that same PVC, as frames pass the remote router’s access link – but the DLCI value may be different. For example, for the PVC between R1 and R2 in Monday’s question, one DLCI value could be used on R1’s access link, and another DLCI value on R2’s access link.
5. A given router’s Frame Relay show commands list the DLCIs that will be used on that router’s access link. These commands do not list the DLCI used on the remote routers’ access links.
6. Frame Relay headers do not use MAC addresses at all.
After reading over these concepts, if you’re thinking you want to change your answer to Monday’s question, go for it! Regardless, now consider this follow-on question: when PC2 replies to PC1, sending a packet to PC1, what DLCI does R2 use when forwarding the packet into the Frame Relay network? And, when the frame arrives on R1’s Frame Relay access link, what DLCI(s) are listed in the frame? Is it possible to know the answer to either question without some command output from router R2? Feel free to comment…
