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Figure 50 Network diagram for inter-AS option A
As shown in 765HFigure 50, 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 that is bound to
the receiving interface, 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. Associating a separate interface with each VPN also requires additional system resources.
452BInter-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.
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