🧰 Cisco MPLS Troubleshooting and Configuration Commands
📘 Overview
This wiki provides a comprehensive guide for Cisco MPLS (Multiprotocol Label Switching) configuration, verification, and troubleshooting.
It covers core MPLS, LDP, IGP integration, MPLS VPN (L3VPN), and Traffic Engineering concepts with practical CLI examples.
🔹 1. MPLS Basics and System Checks
| Command | Description |
|---|---|
show version |
Verifies IOS version and MPLS feature support. |
show ip cef |
Ensures Cisco Express Forwarding is enabled (required for MPLS). |
show mpls interfaces |
Displays interfaces enabled for MPLS. |
show mpls forwarding-table |
Displays label switching table. |
show ip route |
Confirms routing reachability between P/PE routers. |
Tips
- MPLS requires CEF; without it, no label forwarding will occur.
- Ensure
mpls ipis configured on all core-facing interfaces. - Verify loopbacks are reachable across IGP before enabling LDP or RSVP.
🔹 2. MPLS Core Configuration and Verification
Basic MPLS Configuration
router ospf 1
network 10.0.0.0 0.0.0.255 area 0
mpls ip
mpls ldp router-id Loopback0 force
| Command | Description |
|---|---|
show mpls interfaces |
Lists MPLS-enabled interfaces and their states. |
show mpls forwarding-table |
Displays incoming/outgoing label bindings. |
show mpls ldp neighbor |
Shows active LDP neighbors. |
show mpls ldp discovery |
Displays interfaces participating in LDP. |
Troubleshooting
- If no labels → verify
mpls ipon interface. - If LDP neighbor missing → check IGP reachability to LDP router-ID.
- If label not installed → ensure prefix is in IGP table.
🔹 3. Label Distribution Protocol (LDP)
| Command | Description |
|---|---|
show mpls ldp neighbor |
Displays LDP peer information and operational state. |
show mpls ldp discovery |
Shows interfaces sending/receiving LDP Hellos. |
show mpls ldp bindings |
Displays label-to-prefix mappings. |
debug mpls ldp events |
Monitors LDP session establishment and errors. |
mpls ldp router-id Loopback0 force |
Forces use of consistent router ID for LDP sessions. |
Common Problems and Fixes
| Symptom | Likely Cause | Resolution |
|---|---|---|
| LDP neighbor “Oper Down” | Missing IGP route or blocked TCP/646 | Verify IGP reachability and ACLs. |
| Label not advertised | Prefix not in routing table | Check IGP advertisement of prefix. |
| LDP router-ID mismatch | Loopback not reachable | Use same loopback address across peers. |
Verification Example
R1# show mpls ldp neighbor
Peer LDP Identifier: 10.1.1.1:0
TCP connection: 10.1.1.1.646 -> 10.2.2.2.12345
State: Oper; Msgs sent/rcvd: 150/140
🔹 4. MPLS Forwarding Plane
| Command | Description |
|---|---|
show mpls forwarding-table |
Displays label mapping and next-hop info. |
show ip cef <prefix> |
Verifies forwarding entry and label stack. |
traceroute mpls ipv4 <ip> |
Traces MPLS label-switched path. |
ping mpls ipv4 <ip> |
Tests label-switched path end-to-end. |
Troubleshooting Steps
- Verify CEF is enabled (
show ip cef). - Check label bindings (
show mpls ldp bindings). - Compare IGP and MPLS forwarding paths.
- Use
ping mpls ipv4for end-to-end label connectivity testing.
🔹 5. IGP (OSPF / IS-IS) Integration
| Protocol | Command | Description |
|---|---|---|
| OSPF | show ip ospf neighbor |
Displays OSPF adjacencies. |
show ip ospf database |
Verifies advertised LSAs. | |
| IS-IS | show isis neighbors |
Displays IS-IS neighbors. |
show clns interface |
Verifies IS-IS enabled interfaces. |
Tips
- MPLS labels are only assigned to prefixes known via IGP.
- Area mismatches or passive interfaces can block label distribution.
- Confirm Loopbacks are advertised into IGP.
🔹 6. MPLS VPN (Layer 3 VPN)
| Command | Description |
|---|---|
show ip vrf |
Displays configured VRFs. |
show ip route vrf <vrf> |
Displays VRF-specific routing table. |
show bgp vpnv4 all |
Shows VPNv4 routes with labels. |
show ip bgp vpnv4 vrf <vrf> |
Displays VRF-specific BGP routes. |
show mpls forwarding-table vrf <vrf> |
Displays per-VRF label forwarding entries. |
Verification Steps
- Ensure VRF is bound to the correct interfaces.
- Verify import/export route targets.
- Check BGP VPNv4 neighbor activation.
- Confirm route label assignment in forwarding table.
Common Issues
| Problem | Cause | Fix |
|---|---|---|
| No VPN routes | VPNv4 AF not activated | Configure under BGP neighbor. |
| Incorrect RTs | Import/export mismatch | Align RTs between PE routers. |
| Missing labels | Route not labeled | Check BGP label allocation (show bgp vpnv4 all). |
🔹 7. BGP/MPLS Operation
| Command | Description |
|---|---|
show bgp vpnv4 all |
Displays VPNv4 routing table. |
show bgp vpnv4 all summary |
Summarizes BGP VPNv4 peers. |
show bgp vpnv4 all neighbors |
Shows BGP neighbor state and updates. |
debug ip bgp updates |
Monitors live BGP updates. |
Tips
- Ensure
address-family vpnv4is configured under BGP neighbor. - Verify route reflectors advertise VPNv4 routes correctly.
- Use
clear ip bgp <neighbor> soft inafter RT/RD changes.
🔹 8. MPLS Traffic Engineering (RSVP-TE)
| Command | Description |
|---|---|
show mpls traffic-eng tunnels |
Displays active MPLS TE tunnels. |
show mpls traffic-eng topology |
Shows TE topology database. |
show ip rsvp neighbor |
Displays RSVP neighbors. |
show ip rsvp interface |
Lists interfaces with RSVP enabled. |
show ip cef <dest> |
Verifies forwarding path over TE tunnel. |
Troubleshooting
- Tunnel down → RSVP not enabled or IGP missing TE extensions.
- “No path found” → Bandwidth or affinity constraint mismatch.
- Verify head-end configuration and tunnel bandwidth settings.
🔹 9. MPLS OAM (Operations and Maintenance)
| Command | Description |
|---|---|
ping mpls ipv4 <ip> |
Verifies MPLS path end-to-end. |
traceroute mpls ipv4 <ip> |
Traces label-switched hops. |
mpls lsp ping <lsp> |
Tests a specific LSP path. |
show mpls ldp neighbor detail |
Displays detailed LDP statistics. |
Use Case Example
R1# ping mpls ipv4 10.2.2.2
Sending 5, 100-byte MPLS Echos to 10.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)
🔹 10. Logging & Debugging
| Command | Description |
|---|---|
debug mpls ldp events |
Enables LDP-level debug. |
debug mpls forwarding |
Displays data-plane activity. |
debug ip bgp updates |
Shows BGP updates for VPNv4 peers. |
show logging |
Displays logs. |
term mon |
Enables live log output. |
Note: Use undebug all after troubleshooting to prevent CPU overload.
⚙️ 11. Common Troubleshooting Scenarios
🟥 Scenario 1 – LDP Neighbor Down
Steps:
show mpls ldp neighborping <LDP router-id>– test reachability- Check
mpls ldp router-idconsistency - Verify TCP/646 connectivity
🟨 Scenario 2 – No Labels Allocated
Steps:
show ip route– ensure prefix in IGPshow mpls ldp bindings– verify label mappingshow mpls forwarding-table– confirm label installed
🟩 Scenario 3 – VPN Route Missing
Steps:
show bgp vpnv4 all– verify VPNv4 advertisementshow ip vrf– check RT/Import/Exportshow bgp vpnv4 all neighbors– confirm update received
🟦 Scenario 4 – Traffic Blackholed
Steps:
traceroute mpls ipv4 <ip>– trace label hops- Compare IGP vs MPLS forwarding tables
- Check label mismatch between PE routers
🟧 Scenario 5 – MPLS TE Tunnel Down
Steps:
show mpls traffic-eng tunnelsshow ip rsvp interface- Verify IGP TE extensions enabled (
mpls traffic-eng area <id>)
📦 12. References
- Cisco MPLS Command Reference
- Cisco MPLS Fundamentals Guide
- Cisco LDP Configuration Guide
- Cisco Live – MPLS Troubleshooting Deep Dive