PLANET XGS3-24042 - Manual (Page 680)

81-34
In real networking applications, multiple sites of a user VPN may connect to SP with different ASN, or to
different AS of the same SP. Such applications of one VPN crossing multiple autonomy systems are called
Multi-AS VPN. RFC 2547 provides three Multi-AS VPN resolutions:
 VRF-to-VRF：ASBR use VRF interface to create EBGP neighbors and manage VPN routes, which is
also called Inter-Provider Option A;
 EBGP Redistribution of labeled VPN-IPv4 routes ： ASBR use MP-EBGP to advertise label
VPN-IPv4 routes, which is also called Inter-Provider Option B;
 Multihop EBGP redistribution of labeled VPN-IPv4 routes：PE use Multi-hop MP-EBGP to advertise
label VPN-IPv4 routes, which is also called Inter-Provider Option C.
At present we support the first resolution: VRF-to-VRF Multi-VPN resolution.
Multi-VPN resolution
As demonstrated in the nest figure, in this mode, PE routers from two AS directly connects with each other,
and serve as ASBR of the AS they belong to. These PE routers (ASBR) connect with each other via VRF
interfaces, import all RT this system need the other end to learn, export all RT this system want to obtain from
the other, and establish EBGP connections through the VRF interfaces. As a result, the CE they serve will be
able to intercommunicate with and isolate from each other like locating in the same AS, with two PE routers
treating each other as their own CE. Packets will be forwarded within the AS as VPN packets in the 2-layer
label mode, and forwarded as regular IP packets between ASBR.
Figure 81-7 Multi-AS VPN Networking
 The advantage of this Multi-AS VPN mode is easy to realize: no special configuration is needed
between the two PE serving as ASBR.
 The disadvantage is poor extendibility: the PE serving as ASBR need to manage all VPN routes,
and create VPN instances for each VPN. This will cause too many VPN-IPv4 routes on PE.
81.2 BGP MPLS VPN Configuration
BGP MPLS VPN configuration task sequence:
BGP/MPLS backbone
AS 100
CE-1
VPN-1
PE-2
ASBR-1
(PE)
PE-1
CE-3
VPN-1
PE-4
ASBR-2
(PE)
PE-3
CE-4
VPN
Instance
BGP/MPLS backbone
AS 200
MP-IBGP
MP-IBGP
MP-IBGP
MP-IBGP
EBGP
LSP1
VPN LSP1
LSP2
VPN LSP2
IP forwarding
VPN-2
VPN-2
CE-2