HP 10500 SERIES Configuration Manual page 245

Figure 19 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 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
PEs in AS 100 advertise labeled VPN-IPv4 routes to the ASBR PE of AS 100 or the route reflector
1.
(RR) for the ASBR PE through MP-IBGP.
The ASBR PE advertises labeled VPN-IPv4 routes to the ASBR PE of AS 200 through MP-EBGP.
2.
The ASBR PE of AS 200 advertises labeled VPN-IPv4 routes to PEs in AS 200 or to the RR for the
3.
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 20, the routes are advertised through the following steps:
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