3Com S7906E Configuration Manual page 1526

Figure 1-8 Network diagram for inter-provider VPN option B
VPN 1
CE 1
PE 1
MPLS backbone
AS 100
PE 2
VPN LSP 1
CE 2
LSP 1
VPN 2
In terms of scalability, inter-provider VPN option B is better than option A.
When adopting MP-EBGP method, note that:
ASBRs perform no VPN target filtering on VPN-IPv4 routes that they receive from each other.
Therefore, the ISPs in different ASs that exchange VPN-IPv4 routes need to agree on the route
exchange.
VPN-IPv4 routes are exchanged only between VPN peers. A VPN user can exchange VPN-IPv4
routes neither with the public network nor with MP-EBGP peers with whom it has not reached
agreement on the route exchange.
Inter-provider VPN option C
The above two kinds of solutions can satisfy the needs for inter-provider VPNs. However, they require
that the ASBRs maintain and advertise VPN-IPv4 routes. When every AS needs to exchange a great
amount of VPN routes, the ASBRs may become bottlenecks hindering network extension.
One way to solve the above problem is to make PEs directly exchange VPN-IPv4 routes without the
participation of ASBRs:
Two ASBRs advertise labeled IPv4 routes to PEs in their respective ASs through MP-IBGP.
The ASBRs neither maintain VPN-IPv4 routes nor advertise VPN-IPv4 routes to each other.
An ASBR maintains labeled IPv4 routes of the PEs in the AS and advertises them to the peers in
the other ASs. The ASBR of another AS also advertises labeled IPv4 routes. Thus, an LSP is
established between the ingress PE and egress PE.
Between PEs of different ASs, Multi-hop EBGP connections are established to exchange
VPN-IPv4 routes.
ASBR 2
ASBR 1
MP-EBGP
(PE)
(PE)
VPN LSP 3
VPN LSP2
LSP 2
1-11
VPN 1
CE 3
PE 3
MPLS backbone
AS 200
PE 4
CE 4
VPN 2