MPLS LDP Troubleshooting

When faced with a troubleshooting ticket involving MPLS you need to clear the basics first to ensure that everything is working from a Label Distribution perspective.

1. First establish IGP connectivity between all the routers. 
2. Can you ping all the loopbacks?
3. Are there /32 loopbacks configured on every router and advertised into IGP?
4. Is CEF enabled?
5. Is MPLS IP enabled on the required interfaces?
6. Have the routers discovered any LDP neighbors?

I have put together this quick list of commands that I would run on each router to quicky verify that Label Distribution is working

sh run | inc ip cef
sh run | inc mpls ip
sh mpls interface
sh mpls ldp disco
sh mpls ldp neigh

When applied to a router in a working MPLS Core the output should look like below

R21#sh run | inc mpls ip
 mpls ip
 mpls ip
 mpls ip
 mpls ip
 R21#sh run | inc ip cef
 ip cef
 R21#sh run | inc mpls ip
 R21#sh mpls interface
 Interface IP Tunnel Operational
 FastEthernet0/0 Yes (ldp) No Yes
 FastEthernet0/1 Yes (ldp) No Yes
 FastEthernet1/0 Yes (ldp) No Yes
 FastEthernet2/0 Yes (ldp) No Yes
 R21#sh mpls ldp neigh
 Peer LDP Ident: 22.22.22.22:0; Local LDP Ident 21.21.21.21:0
 TCP connection: 22.22.22.22.24740 - 21.21.21.21.646
 State: Oper; Msgs sent/rcvd: 294/292; Downstream
 Up time: 04:02:51
 LDP discovery sources:
 FastEthernet0/1, Src IP addr: 172.16.1.2
 Addresses bound to peer LDP Ident:
 172.16.1.0 172.16.1.2 22.22.22.22
 Peer LDP Ident: 172.16.1.9:0; Local LDP Ident 21.21.21.21:0
 TCP connection: 172.16.1.9.51621 - 21.21.21.21.646
 State: Oper; Msgs sent/rcvd: 210/213; Downstream
 Up time: 02:48:22
 LDP discovery sources:
 FastEthernet1/0, Src IP addr: 172.16.1.9
 Addresses bound to peer LDP Ident:
 172.16.1.9 24.24.24.24 172.16.1.10
 Peer LDP Ident: 26.26.26.26:0; Local LDP Ident 21.21.21.21:0
 TCP connection: 26.26.26.26.43548 - 21.21.21.21.646
 State: Oper; Msgs sent/rcvd: 118/119; Downstream
 Up time: 01:27:44
 LDP discovery sources:
 FastEthernet2/0, Src IP addr: 172.16.1.15
 Addresses bound to peer LDP Ident:
 172.16.1.15 26.26.26.26 172.16.1.13
 Peer LDP Ident: 23.23.23.23:0; Local LDP Ident 21.21.21.21:0
 TCP connection: 23.23.23.23.62234 - 21.21.21.21.646
 State: Oper; Msgs sent/rcvd: 28/28; Downstream
 Up time: 00:11:03
 LDP discovery sources:
 FastEthernet0/0, Src IP addr: 172.16.1.4
 Addresses bound to peer LDP Ident:
 172.16.1.4 23.23.23.23 172.16.1.6
 R21#sh run | inc ip cef
 ip cef
 R21#sh run | inc mpls ip
 R21#sh mpls interface
 Interface IP Tunnel Operational
 FastEthernet0/0 Yes (ldp) No Yes
 FastEthernet0/1 Yes (ldp) No Yes
 FastEthernet1/0 Yes (ldp) No Yes
 FastEthernet2/0 Yes (ldp) No Yes
 R21#sh mpls ldp disco
 Local LDP Identifier:
 21.21.21.21:0
 Discovery Sources:
 Interfaces:
 FastEthernet0/0 (ldp): xmit/recv
 LDP Id: 23.23.23.23:0
 FastEthernet0/1 (ldp): xmit/recv
 LDP Id: 22.22.22.22:0
 FastEthernet1/0 (ldp): xmit/recv
 LDP Id: 172.16.1.9:0; no host route
 FastEthernet2/0 (ldp): xmit/recv
 LDP Id: 26.26.26.26:0

This LDP Troubleshooting was taken from an MPLS L3VPN Tutorial which I posted here – MPLS L3VPN Configuration Tutorial –

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Roger Perkin - CCIE #50038 is a Network Automation Engineer & CCIE Consultant based in the UK, currently working for Softcat Plc as a Senior Network & Security Consultant.
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