Operations, Troubleshooting and Verification
What this domain tests
Operations, Troubleshooting and Verification tests whether you can isolate control-plane versus data-plane faults. You should know which show commands prove a label binding, which prove an LFIB entry, and how to test a VRF path end to end.
Core rules
- Verify the control plane first: LDP bindings and sessions, RSVP-TE sessions and reservations, MP-BGP VPNv4 routes, and VRF route tables.
- Verify the data plane with the MPLS forwarding table, label traceroute, and VRF-aware ping. A route can exist without a usable label or adjacency.
- Check MTU and label stack depth. Extra labels can push a packet over the underlay MTU, causing drops that look like application failures.
- Use VRF-aware commands; a PE ping without the VRF may use the global table and give a misleading result.
- Compare both directions. MPLS forwarding is label-switched per direction, and a return path can fail independently.
Worked example
A customer cannot ping from CE-A to CE-B. On PE1, the VRF route to CE-B exists and the VPNv4 route is present. The LFIB has no entry for the VPN label. Checking MP-BGP shows the route was received but the next hop is unreachable. The ingress PE therefore cannot resolve the transport LSP. The fault is control-plane next-hop resolution, not the customer interface. After fixing the underlying route, the LFIB entry appears and the ping succeeds.
Traps
Do not stop at a VRF route; check the label and next hop. Do not ping from the global table when testing a VRF. Do not ignore asymmetry or MTU. Do not assume a missing label means LDP is down; MP-BGP or RSVP may be the source.