Mpls Lsp Ping

MPLS LSP ping is the name for an MPLS echo request and MPLS echo reply. Ping is a well-known troubleshooting tool for IP networks that is used to figure out if the object is there. If it is, you see an echo. It is like using SONAR on a submarine. Ping uses ICMP, which was designed to augment the IP protocol because it can signal error conditions (destination unreachable, time exceeded, and so on) and send informational advertisements (redirect, address mask, and so on). Ping uses ICMP to carry echo request and echo reply packets. The echo request packet is sent toward the destination, which should then reply with an echo reply packet. The source receiving the echo reply indicates that the two hosts can see each other on the network level (Layer 3).

Because MPLS cannot work without IP on the network level, you can still use the IP ping when the network is running MPLS. The ping packets are labeled and label-switched throughout the network. Why invent MPLS LSP ping? Well, IP ping is insufficient for verifying the correctness of the MPLS LSP. Although it can verify whether the connectivity is present on the IP level, it does not verify whether the LSP is broken. If you have a plain IP-over-MPLS network and LDP is broken between two LSRs, ping indicates that there is no problem as the echo request makes it to the destination and the echo reply makes it back to the source. Between the two LSRs where the LDP session is broken, the packets are no longer labeled. The ping indicates falsely that everything is okay, when in reality the LSP is broken.

To see that this can lead to useless troubleshooting, imagine that you are switching Any Transport over MPLS (AToM) traffic across that LSP, and the two LSRs with the broken LDP session are P routers in an AToM network. Refer to Figure 14-3 for the network.

Figure 14-3 Broken LSP in AToM Network

AtoM Service

Broken LDP Session

Ping: Echo Request

Ping: Echo Request

Ping: Echo Reply

Ping: Echo Reply

If the LSP is broken, the AToM traffic becomes unlabeled between LSRs P2 and P3. Because those two LSRs do not know how to forward those frames, they are dropped. A ping from PE to PE router across the LSP would be successful, but the AToM traffic would fail. To have a similar protocol to the IP ping protocol indicate specific problems with MPLS LSPs, MPLS LSP ping was invented.

LSPs can break for any number of reasons, while the IP connectivity remains fine. Following are some reasons an LSP could be broken:

■ MPLS is not enabled on an LSR (or one interface).

■ The LFIB has a wrong entry for that LSP (wrong in/out label or wrong outgoing next-hop information).

■ The software and hardware LFIB have a discrepancy.

For some of these problems, the packets become unlabeled; others are label-switched, but in a wrong way. That is why you need a mechanism to test the LSP end to end and give some helpful feedback when the LSP is broken. When you are troubleshooting the LSP, it is good to know where the LSP is broken and what the error is. MPLS LSP ping detects problems in the forwarding plane, but it also checks the control plane against the information in the data plane.

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