Question

Two IOS-XR routers, PE1 and PE2, are directly connected over a point-to-point link and configured for IS-IS Level-2. The adjacency repeatedly cycles between INIT and DOWN, never reaching UP. Logs on PE1 show IIH packets received from PE2, but PE2 never sees PE1's hellos acknowledged. PE1's interface MTU is 1500; PE2's interface MTU was recently changed to 1400 during a maintenance window. What is the most likely cause of the failure?

Accepted Answer

IS-IS pads its hello PDUs to the full interface MTU by default, so the 1500-byte padded hellos from PE1 are dropped by PE2's smaller 1400-byte MTU, preventing the adjacency from forming.

Answer

The two routers have mismatched IS-IS system IDs, which prevents the three-way handshake from completing on the point-to-point link.

Answer

One router is configured for narrow metrics and the other for wide metrics, causing the hellos to be silently discarded.

Answer

The area addresses differ between PE1 and PE2, which is required to match for Level-2 adjacencies to form.