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 Topics ECMP Labels for MPLS Overview on page 242•
• Verifying and Troubleshooting MPLS Connectivity on page 370
• Packet Flow Examples for Verifying MPLS Connectivity on page 372
• ping mpls ip
245Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview
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