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ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter16 Configuring Tag Switching and MPLS
MPLS Overview
Figure16-5 Summary Route Propagation Between LSRs
LFIB Table Look Up Process
Figure 16-6 shows the packet transmission and LFIB table look up process used between a source and
destination over an ATM MPLS network. AdminLSR1 is the ingress point for packets from the r outer
AdminRt1. When the LSR receives the packet it determines the FEC and determines the LSP to use by
looking in the LFIB table.
Note The LFIB table is propagated using the LDP discover mechanism shown in Figure16-5.
AdminLSR1 adds the label (VPI/VCI) 65,180 to the packet and forwards the packet out ATM interface
0/1/0. The intermediate LSR (NetLSR2) takes the labeled packet and pairs the incoming interface and
label and then uses a lookup table to determine the outgoing interface and label. After swapping the
incoming label with the new outgoing label the packet is forwarded out to the next LSR.
The label swapping process is continued at each LSR until the last LSR. The egress LSR performs the
same look up as the intermediate LSRs but the outgoing label is stripped off and the packet is either
routed or switched using Layer 3 to its destination.
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