Question 1
Q1A service provider's core network utilizes OSPF as the IGP and LDP for label distribution. An engineer adds a new PE router, PE3, which successfully establishes OSPF adjacencies and LDP sessions with its neighbors. However, PE3 is unable to build a label-switched path (LSP) to the remote PE1 router. The command show ldp session on PE3 confirms an active session to PE1's loopback address (192.168.255.1). Despite this, the show route table mpls.0 output on PE3 is empty. Which of the following is the most likely cause for this issue?
Show answer & explanation
Correct answer: B
LDP relies on the underlying IGP (OSPF in this case) to resolve the path to the remote LDP peer's transport address (typically the loopback). LDP uses this route from inet.0 to install the LSP information into the inet.3 table. If the route to the remote peer's loopback is missing from inet.0, LDP cannot resolve the next hop, and no label information will be installed in mpls.0. Even though the LDP session is up (likely via directly connected interfaces), the LSP for the remote FEC cannot be built without the IGP route.
