Question

Two directly connected PE routers, PE1 and PE2, have MPLS LDP enabled on their interconnecting interface. LDP link Hello adjacency forms successfully, but the LDP session never reaches the OPERATIONAL state. On PE1, 'show mpls ldp discovery' shows PE2's Hellos being received, yet 'show mpls ldp neighbor' lists no established neighbor. PE1 uses transport address 10.0.0.1 (Loopback0) and PE2 uses 10.0.0.2 (Loopback0), but neither router has a route to the other's loopback. What is the most likely cause of the failure?

Accepted Answer

The IGP is not advertising the loopback addresses used as LDP transport addresses, so the TCP session for the LDP session cannot be established

Answer

The LDP Hello hold time is mismatched between PE1 and PE2, preventing neighbor discovery

Answer

LDP label distribution mode must be changed from downstream unsolicited to downstream on demand for the session to come up

Answer

The interface MTU values differ, causing the LDP session negotiation to fail