HP HSR6800 Configuration Manual page 254

Figure 71 Network diagram for inter-AS option A
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. Moreover, the requirement to create a separate subinterface for each VPN also calls for
higher performance of the PEs.
Inter-AS option B
In this kind of solution, two ASBRs use MP-EBGP to exchange labeled VPN-IPv4 routes that they
have obtained from the PEs in their respective ASs.
As shown in
1. PEs in AS 100 advertise labeled VPN-IPv4 routes to the ASBR PE of AS 100 or the route
reflector (RR) for the ASBR PE through MP-IBGP.
2. The ASBR PE advertises labeled VPN-IPv4 routes to the ASBR PE of AS 200 through
MP-EBGP.
3. The ASBR PE of AS 200 advertises labeled VPN-IPv4 routes to PEs in AS 200 or to the RR for
the PEs through MP-IBGP.
For more information about RR, see Layer 3—IP Routing Configuration Guide.
The ASBRs must perform the special processing on the labeled VPN-IPv4 routes, which is also
called ASBR extension method.
Figure 72, the routes are advertised through the following steps:
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