Nov 21 2008
Inter AS MPLS VPN Option B, 2a – Packet Capture
I’ve setup the topology below.
I configured a loopback address with ip address 10.10.10.10 on PE1 and put it inside the VRF for VPN A. One PE1 I could see the VPN label being generated by BGP as below.
sh ip bgp vpnv4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 65300:1 (CUST)
10.10.10.10/32 0.0.0.0 19/nolabel
As you can see the “In label” is 19. To confirm the label is changed at different points in the LSP path I checked the label for 10.10.10.10 on ASBR1, ASBR2 and PE2, see below for output.
ASBR1
ASBR1#sh ip bgp vpnv4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 65300:1
10.10.10.10/32 1.1.1.1 17/19
As you can see ASBR1 has generated an “In label” of 17 which it advertises to ASBR2.
ASBR2
ASBR2#sh ip bgp vpnv4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 65300:1
10.10.10.10/32 192.168.1.1 23/17
ASBR2 generates its own “In label” of 23 and advertises it to PE2, please see below.
PE2
PE2#sh ip cef vrf CUST 10.10.10.10
10.10.10.10/32
nexthop 11.0.0.5 FastEthernet1/0 label 16 23
PE2#sh ip bgp vpnv4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 65300:1
10.10.10.10/32 4.4.4.4 nolabel/23
I also ran a packet capture on the ASBR2 interface which peers with ASBR1 and sure enough when I ran a ping to 10.10.10.10 I saw a single stacked MPLS frame, with a VPN label of 17.
The reason there is only a single label on the frame between ASBR2 and ASBR1, can be explained due to the next hop changing.
Therefore ASBR2 is the penultimate hop in the LSP and pops the top label and hence you see the VPN label alone on the wire.
