Ldp Discovery Mechanisms - Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual

JUNOSe 11.0.x BGP and MPLS Configuration Guide
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

LDP Discovery Mechanisms

LDP uses two different mechanisms for peer discovery. Peer discovery removes the
need to explicitly configure the label-switching peers for an LSR.
236
LDP Discovery Mechanisms
ECMP Labels for MPLS on page 233
Verifying and Troubleshooting MPLS Connectivity on page 363
Packet Flow Examples for Verifying MPLS Connectivity on page 365
ping mpls ip
ping mpls l2transport
ping mpls l3vpn
ping mpls rsvp tunnel
ping mpls vpls
trace mpls ip
trace mpls l2transport
trace mpls l3vpn
trace mpls rsvp tunnel
trace mpls vpls