Information Technology Reference
In-Depth Information
Example 10-17 shows the prefix information on PE3, where 192.168.67.7 is a next-hop P
router (not shown in Figure 10-16) and 192.168.100.4 is PE4's loopback 0 address. Two
labels are imposed: The top label 20 is the IGP label to reach PE4, and the bottom label 20
is the VPN label, Lv1. The advertisement of the IGP prefixes and labels is as shown previ-
ously in the section “Understanding MPLS Fundamentals.”
Example 10-17
Prefix and Label Information for 172.16.0.0 on PE3
PE3#show ip cef vrf VPNa 172.16.0.0 detail
172.16.0.0/16, version 9, epoch 0, cached adjacency 192.168.67.7
0 packets, 0 bytes
tag information set
local tag: VPN-route-head
fast tag rewrite with Et0/0, 192.168.67.7, tags imposed: {20 20}
via 192.168.100.4, 0 dependencies, recursive
next hop 192.168.67.7, Ethernet0/0 via 192.168.100.4/32
valid cached adjacency
tag rewrite with Et0/0, 192.168.67.7, tags imposed: {20 20}
The prefix is imported on PE3 into the VRF VPN A, because the VRF is configured to
import prefixes with an RT of 200:200. As is done on a typical PE, the VPNv4 prefix is then
converted to IPv4. Because PE3 considers PE2 to be a CE, the IPv4 prefix is advertised.
Example 10-18 shows the prefix information on PE2, where 192.168.100.2 is PE1 and
192.168.56.6 is PE3.
Example 10-18
Output of
show ip route vrf VPNa
on PE2
PE2#show ip bgp vpnv4 all 172.16.0.0
BGP routing table entry for 100:100:172.16.0.0/16, version 9
Paths: (1 available, best #1, table VPNa)
Advertised to non peer-group peers:
192.168.100.2
200 65000
192.168.56.6 from 192.168.56.6 (192.168.100.6)
Origin IGP, localpref 100, valid, external, best
Extended Community: RT:100:100
PE2 treats the prefix received from PE3 just like a prefix from a CE. The same process is
repeated on PE2 as in PE4. When the prefix is advertised to PE1, the local RD for the VRF
VPNa is prepended with an RT of 100:100. A new label, Lv2, is assigned for the VPNv4
prefix. A process similar to PE3 is performed on PE1. CE1 finally receives the IPv4 prefix.
Example 10-19 shows the prefix information on PE1, where 192.168.23.3 is a next-hop P
router and 192.168.100.5 is PE2. Note that label 21 is the new VPN label, Lv2.
