The Troubleshooting Of MPLS Layer 3 VPNs

I would like to dedicate this post to MPLS L3 VPNs troubleshooting and more particularly using the Traceroute command. It can be sometimes difficult to find out where is the issue when testing connectivity between sites attaches to a MPLS VPN backbone. I will explain the behavior of Traceroute in MPLS VPN environment which is quite different than in “classical” IP environment. Finally I will talk about the MPLS LSP Ping feature and how to use it to detect break in MPLS LSP.

Topology:

I am using almost the same topology as the one used in this post: Basic MPLS. I have just added a P router.

 Platform/IOS:

 Cisco 2691/12.4(15)T11 Adv IP services

 First of all if you want to get a basic understanding of MPLS and MPLS Layer 3 VPNs I suggest to read my previous posts: Basic MPLS and MPLS Layer 3 VPNs

 The following is already configured:

    MP-iBGP between R2 and R5
    EBGP between R2 and R1, R5 and R6
    OSPF area 0 between R2,R3,R4,R5
    LDP between R2,R3,R4,R5
    R3 and R4 are P routers and R2 and R5 are PE routers
    R1 is representing the CE device at Site A announcing its loopback address 1.1.1.1/32
    R2 is representing the CE device at Site B announcing its loopback address 6.6.6.6/32

 In this troubleshooting scenario we want to focus on the loopbacks reachability between Site A and Site B.