| 1:
| What is the innermost label in the label stack of a packet sent over an AToM pseudowire? |
| A1:
| Answer: The innermost label in the label stack is the VC label. This label is used by the egress PE router to associate the Layer 2 PDU carried in the packet with the correct attachment circuit. |
| 2:
| How do the peer PE routers discover each other? |
| A2:
| Answer: They use extended (unicast UDP port 646) LDP discovery. |
| 3:
| How do the peer PE routers advertise VC labels to each other? |
| A3:
| Answer: They form an LDP neighbor relationship and exchange label mapping messages. |
| 4:
| If an attachment circuit changes state to down, how does the PE router indicate this state change to its peer PE router? |
| A4:
| Answer: It sends an LDP label withdraw message. |
| 5:
| How do LDP LSRs with directly connected interfaces discover each other? |
| A5:
| Answer: They use UDP port 646 and the all routers multicast address (224.0.0.2). |
| 6:
| How do LSRs with directly connected interfaces establish an LDP session? |
| A6:
| Answer: They use TCP unicast communication (port 646) between LDP ID (transport) addresses. |
| 7:
| Assuming that the PE routers are not in a back-to-back configuration (or directly connected via a GRE or L2TP tunnel), what is the minimum number of labels required to transport Layer 2 PDUs across an MPLS network between ingress and egress PE routers? |
| A7:
| Answer: The minimum number of labels is two (tunnel and VC labels). |
| 8:
| How many LSPs are required to establish a single AToM pseudowire between peer PE routers? |
| A8:
| Answer: Two LSPs are required (LSPs are unidirectional). |
| 9:
| What is a likely cause if labels are not locally assigned to prefixes? |
| A9:
| Answer: A likely cause is that CEF is disabled. |
| 10:
| Assuming an LDP signaled tunnel LSP, to what prefix does the tunnel (IGP) label correspond? |
| A10:
| Answer: It corresponds to the egress PE router's LDP ID address. |
