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    MPLS Connectivity Verification and Troubleshooting Methods

    You can use the MPLS ping application to examine the network reachability and identify any broken links for diagnostic purposes. Before using the ping MPLS feature, make sure that the receiving interface on the VPN or LSP remote endpoint has MPLS enabled, and that the loopback interface on the outbound node is configured as 127.0.0.1. The source address for MPLS probes must be a valid address on the device. When you use the ping MPLS feature from a J Series device operating as the inbound (ingress) node at the entry point of an LSP or VPN, the router sends probe packets into the LSP or VPN. Based on how the LSP or VPN outbound (egress) node at the remote endpoint of the connection replies to the probes, you can determine the connectivity of the LSP or VPN. Each probe is an echo request sent to the LSP or VPN exit point as an MPLS packet with a UDP payload. If the outbound node receives the echo request, it checks the contents of the probe and returns a value in the UDP payload of the response packet. If the device receives the response packet, it reports a successful ping response. Responses that take longer than 2 seconds are identified as failed probes.

    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 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.

    For L3VPN services, the ping mpls l3vpn command is used to examine the operability of a MPLS Layer 3 VPN connection. For VPLS routing instances, the ping vpls instance command is used to examine the reachability of a VPLS instance. The ping vpls instance command uses a difference command structure and operates in a different fashion than the ping mpls command used for VPNs and Layer 2 circuits. For point-to-point services, the pseudowire ping mechanism is used to verify the network accessibility and identify any problems in the link.

    Modified: 2017-07-20