MPLS VPN Technology

Practice
VPN Label Label Propagation Propagation (Cont.) (Cont.)
CE-router
MPLS VPN Backbone
Ingress-PE P-router P-router Egress-PE
CE-router CE-router
Step #3: Label stack is built in Virtual Forwarding table
CE-router
Ingress-PE#show ip cef vrf Vrf1 203.1.20.0 detail
203.1.20.0/24, version 57, cached adjacency to Serial1/0.2
0 packets, 0 bytes
tag information set
local tag: VPN-route-head
fast tag rewrite with Se1/0.2, point2point, tags imposed: {26 38}
via 192.168.3.103, 0 dependencies, recursive
next hop 192.168.3.10, Serial1/0.2 via 192.168.3.103/32
valid cached adjacency
tag rewrite with Se1/0.2, point2point, tags imposed: {26 38}
© 2002, Cisco Systems, Inc. www.cisco.com MPLS v2.1-102
Step 3 The ingress PE-router has two labels associated with a remote VPN
route – a label for BGP next-hop assigned by the next-hop P-router via
LDP (and taken from local Label Information Base – LIB) as well as the
label assigned by remote PE-router and propagated via MP-BGP update.
Both labels are combined in a label stack and installed in the virtual
forwarding (VRF) table.
The label stack in the virtual forwarding table can be inspected with the show ip
cef vrf detail command. The tags imposed part of the printout displays the MPLS
label stack. The first label in the MPLS label stack is the TDP/LDP label toward
the egress PE-router and the second label is the VPN label advertised by the
egress PE-router.
Q1) Which router assigns the VPN label?
A) Every PE- or P-router in the path generates its own local label.
B) The advertising CE-router also assigns the label.
C) The egress PE-router assigns the VPN label.
D) The ingress PE-router assigns the VPN label.
Copyright © 2002, Cisco Systems, Inc. MPLS VPN Technology 115