MPLS Connectivity Verification and Troubleshooting Methods

In IP networks, the ping and traceroute commands enable you to verify network connectivity and find broken links or loops. In MPLS-enabled networks, you can use the ping command to determine whether IP connectivity exists to a destination even when the ping packets must traverse multiple LSPs. You can use the traceroute command to determine the labels that data packets use when traversing LSPs to the destination.

In an MPLS-enabled network, however, you cannot use these IP commands to determine MPLS connectivity to a destination.

You can use the MPLS ping and trace features to detect data plane failures in LSPs. Specific mpls ping and trace mpls commands enable you to target different types of MPLS applications and network topologies. The various ping mpls and trace mpls commands send UDP packets, known as MPLS echo requests, to the egress LSR of MPLS packets in a given FEC. Each echo request is forwarded along the same data path as the MPLS packets in that FEC.

The echo request packets use a destination address in the 127.0.0.0/8 range and port 3503. The default address is 127.0.0.1. This address range prevents IP from forwarding the packet, so that the echo request must follow the MPLS data path. This behavior is different from that of the IP ping and traceroute commands, which send ICMP packets to the actual destination.

Each MPLS echo request packet contains information about the FEC stack that is being validated. LSRs that receive an MPLS echo request respond with MPLS echo reply packets. (Even when MPLS is not enabled on that router, echo reply packets are sent by E Series routers that receive an echo request packet. This situation is a transient condition when the router is receiving labeled packets. A return code in the echo replies indicates to the sending router that no label mapping exists on the receiving router.)

The ping mpls commands perform a basic connectivity check. When the echo request exits the tunnel at the egress LSR, the LSR sends the packet to the control plane. The egress router validates the FEC stack to determine whether that LSR is the actual egress for the FEC. The egress router sends an echo reply packet back to the source address of the echo request packet. The egress router can send the packet back by means of either the IP path or the MPLS path.

The trace mpls commands isolate faults in the LSP. For these commands, successive echo request packets are sent along the path. The first packet has a TTL of one; the TTL value is incremented by one for each successive packet. The first packet therefore reaches only the next hop on the path; the second packet reaches the next router after that. Echo request packets are sent until either an echo reply is received from the egress router for the FEC or a TTL of 32 is reached.

When a TTL expires on an LSR, that LSR sends an echo reply packet back to the source. For transit routers, the echo reply indicates that downstream mapping exists for the FEC, meaning that the packet would have been forwarded if the TTL had not expired. The egress router sends an echo reply packet verifying that it is the egress.

Although you cannot send IPv6 UDP packets for MPLS ping, you can use the ping mpls l3vpn command with an IPv6 prefix to investigate IPv6 VPNs.

Related Documentation