168
•
Multihop EBGP redistribution of labeled VPN-IPv4 routes
between PE routers
—PEs advertise
VPN-IPv4 routes to each other through MP-EBGP. This solution is also called inter-AS option C.
Inter-AS option A
In this solution, PEs of two ASs are directly connected through multiple subinterfaces, and each PE is also
the ASBR of its AS. Each PE treats the other as a CE and advertises unlabeled IPv4 unicast routes through
EBGP. The PEs associate a VPN instance with at least one subinterface.
Figure 46
Network diagram for inter-AS option A
As shown in
, VPN 1 routes are advertised from CE 1 to CE 3 by using the following process:
1.
PE 1 advertises the VPN routes learned from CE 1 to ASBR 1 through MP-IBGP.
2.
ASBR 1 performs the following operations:
a.
Adds the routes to the routing table of the VPN instance whose import target attribute matches
the export target attribute of the routes.
b.
Advertises the routes as IPv4 unicast routes to its CE (ASBR 2) through EBGP.
3.
ASBR 2 adds the IPv4 unicast routes to the routing table of the VPN instance bound to the receiving
subinterface, and advertises the routes to PE 3 through MP-IBGP.
4.
PE 3 advertises the received routes to CE 3.
Packets forwarded within an AS are VPN packets that carry two labels. Packets forwarded between
ASBRs are common IP packets.
Inter-AS option A is easy to carry out because no special configuration is required on the PEs acting as
the ASBRs.
However, it has limited scalability because the PEs acting as the ASBRs must manage all the VPN routes
and create VPN instances on a per-VPN basis. This leads to excessive VPN-IPv4 routes on the PEs.
Creating a separate subinterface for each VPN also requires additional system resources.
Inter-AS option B
In this solution, two ASBRs use MP-EBGP to exchange VPN-IPv4 routes that they obtain from the PEs in
their respective ASs.
