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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
2-17
As shown in the preceding figure, the VRFs between the ASBRs set up common EBGP sessions
to exchange IPv4 routes and the ASBRs and PEs set up MP-IBGP sessions to exchange VPN
routes. For the VRF on an ASBR, the other VRF, with which the EBGP session is set up, is
equivalent to a CE. This configuration scheme is similar to the common intra-domain scheme.
The ASBRs and PEs set up MP-IBGP sessions to exchange VPN routes. The VRFs of ASBRs
set up EBGP sessions in the BGP VRF address family mode to exchange IPv4 routes.
Characteristics and limitations
The VRF-to-VRF mode is easy to implement by directly using MP-IBGP. The service deployment
is also simple. This scheme, however, requires an interface (generally a logical sub-interface) for
each inter-AS VPN on an ASBR. The number of bound interfaces at least should be equal to the
number of inter-AS VPNs. You should configure an interface for each VPN on the ASBR,
complicating network expansion. In addition, the separate creation of sub-interfaces for each
VPN poses high requirements on ASBRs. As a result, this scheme is generally applicable to
networks with a small number of inter-AS VPNs.
The configuration of OptionA is similar to that of a BGP/MPLS VPN and is not described here.
2.3.2.2 OptionB: Single-Hop MP-EBGP Mode
In the OptionA scheme, you need to configure a VRF for each VPN on an ASBR and bind the
VRF to an interface. This is because VPN routes cannot be directly transmitted between EBGPs
and can only be carried through MP-IBGP. If the VPN routes can be directly transmitted between
EBGPs, you are not required to configure VRFs on the ASBR. This is clearly a better
implementation mode. In this case, the OptionB scheme extends MP-IBGP and allows the direct
transmission of VPN routes between ASBRs. This is called the single-hop EBGP, as shown in
the following topology.
Figure 9 OptionB inter-AS VPN
Characteristics and limitations