MPLS troubleshooting usually comes down to:

• Label distribution (LDP/RSVP)

• Label forwarding (LFIB/ILM/NHLFE)

• Underlying IGP reachability (OSPF/ISIS)

• End-to-end ping/traceroute using labels

MPLS Troubleshooting Metholodogy

1. Verify IGP Reachability (MPLS depends on IGP)

   1. Cisco:

      1. ping <loopback>

      2. traceroute <loopback>

      3. show ip route <loopback>

   2. Nokia:

      1. ping <loopback>

      2. traceroute <loopback>

      3. show router route-table <loopback>

   3. if loopbacks are not reachable, fix IGP before MPLS.

2. Check MPLS Enabled on interfaces

   1. Cisco: show mpls interfaces

   2. Nokia: show router mpls interface

   3. Ensure MPLS is enabled on WAN links (not just loopbacks)

3. Check Label Distribution Protocol (LDP/RSVP)

   1. Cisco (LDP)

      1. show mpls ldp neighbor

      2. show mpls ldp bindings

   2. Nokia (LDP)

      1. show router ldp session

      2. show router ldp bingings

   3. If session DOWN --> Check TCP/646 reachabilit, IGP path, ACLS.

4. Check Label Forwarding

   1. Cisco: show mpls forwarding-table

   2. Nokia:

      1. show router mpls lsp

      2. show router mpls fwd

   3. Each FEC (prefix) should have an outgoing Label.

5. End-to-End Verification

   1. Cisco:

      1. ping mpls ipv4 <destination>

      2. traceroute mpls ipv4 <destination>

   2. Nokia:

      1. oam mpls ping lsp <name>

      2. oam mpls traceroute lsp <name>

MPLS Checklist (with Commands)

1. MPLS not enabled on link

   1. Cisco: show mpls interfaces

   2. Nokia: show router mpls interface

   3. Fix: mpls ip (Cisco) or mpls-enable (Nokia)

2. LDP/RSVP session not up

   1. Cisco: show mpls ldp neighbor

   2. Nokia: show router ldp session

   3. Check: IGP reachability, TCP/646, Router-ID Consistency.

3. No label for prefix

   1. Cisco: show mpls ldp bindings

   2. Nokia: show router ldp bindings

   3. If missing --> not in IGP or label filtering in place

4. Traffic blackhole (no LFIB entry)

   1. Cisco: ping mpls ipv4 <dest-loopback>

   2. Nokia: oam mpls ping lsp <name>

   3. If fails, run traceroute to see where labels are missing.

Common Mistakes / Pitfalls

• MPLS not enabled on link (forgetting mpls ip or mpls-enable)

• Router ID mismatch between LDP peers.

• LDP not running on all IGP links --> blackholes.

• MTU Mismatch --> MPLS labels add overhead, drops if MTU too small.

• IGP not carrying loopbacks --> LDP session never forms.

• Label filtering / policy --> prevents label distribution.

Quick Fix recipes

• LDP Session DOWN

  • Cisco: show mpls ldp neighbor

  • Nokia: show router ldp session

  • Check: ping router-IDs, TCP/646 open, IGP router present.

• No Label for prefix

  • Cisco: show mpls ldp bindings <prefix>

  • Nokia: show router ldp bindings <prefix>

  • Ensure prefix in IGP, enable MPLS on interfaces.

• MPLS ping fails

  • Cisco: traceroute mpls ipv4 <loopback>

  • Nokia: oam mpls traceroute lsp <name>

  • Follow label stack untill break point.

• Traffic blackhole:

  • Cisco: show mpls forwarding-table

  • Nokia: show router mpls fwd

  • Lookf or "No label" --> Fix LDP distribution or IGP