H3C Technologies H3C S12500-X Series Switches - Manual (Page 118)

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Figure 35 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.
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 labeled VPN-IPv4 routes that they obtain from the
PEs in their respective ASs.
As shown in
Figure 36 , the routes are advertised by using the following process:
1. PEs in AS 100 advertise labeled VPN-IPv4 routes to the ASBR-PE of AS 100 or the route reflector
(RR) of 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 of the PEs
through MP-IBGP.
The ASBRs must perform special processing on the labeled VPN-IPv4 routes, which is also called ASBR
extension method.