Troubleshooting guide

8-15
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 8 Troubleshooting Tag and MPLS Switching Connections
MPLS Overview
Figure 8-2 Summary Route Propagation Between LSRs
Figure 8-2 shows the LDP discover mechanism used to periodically transmit LDP Hello messages, and
to signal it is ready to advertise label bindings. The LSR sends the LDP Hello messages as UDP packets
to the well known LDP port (646).
The Hello messages carry the LDP identifier, or “ID”, of the label space that the sending LSR wants to
advertise, as well as other information. In Figure 8-2, SalesLSR4 sends a hello packet with the VPI and
VCI used to connect to FEC 172.68.0.0. Each LSR then propagates that FEC, replacing the VPI and VCI
used to connect to its ingress interface.
When a labeled packet is being sent from an LSR to its neighbor LSR, the label value carried by the
packet is the label value that the egress LSR assigned to represent the FEC of the packet. This causes the
label value (VPI/VCI) to be swapped as the packet traverses the network.
Figure 8-3 shows the packet transmission and LFIB table look up process used between a source and
destination over an ATM MPLS network.
a0/0 a0/0/0 a0/1/0
e1/0
a2/0/0
a1/1/0
a1/0/0
a2/1/0
a3/0/0
e3/1/0
e3/2/0
68272
AdminLSR1
NetLSR2
NetLSR3
SalesRt1
172.68.10/24
SalesLSR4
e2/0
SalesRt2
172.68.44/24
AdminRt1
Use label "implicit-null"
for FEC 172.68/16
Use label "85,220"
for FEC 172.68/16
Use label "65,180"
for FEC 172.68/16