Juniper BGP, Configuration Manual

Page: 272 / 748

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 on page 233
■
Verifying and Troubleshooting MPLS Connectivity on page 367
■
Packet Flow Examples for Verifying MPLS Connectivity on page 369
■
ping mpls ip
■
ping mpls l2transport
236 ■
MPLS Connectivity Verification and Troubleshooting Methods
JUNOSe 11.1.x BGP and MPLS Configuration Guide