HP HSR6800 Configuration Manual page 263

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Configuration of OSPF areas between a PE and a CE
The OSPF area between a PE and a CE can be either a non-backbone area or a backbone
area.
In the OSPF VPN extension application, the MPLS VPN backbone is considered the backbone
area (area 0). The area 0 of each VPN site must be connected to the MPLS VPN backbone
because OSPF requires that the backbone area be contiguous.
If a VPN site contains an OSPF area 0, the connected PE must be connected to the backbone
area of the VPN site through area 0. You can configure a logical connection by using a virtual
link.
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BGP/OSPF interaction
PEs advertise VPN routes to each other through BGP and to CEs through OSPF.
Conventional OSPF considers two sites to be in different ASs even if they belong to the same
VPN. Therefore, the routes that one site learns are advertised to the other as external routes.
This results in more OSPF traffic and network management problems.
The extended OSPF protocol supports multiple instances to address the previous problems.
Properly configured, OSPF sites are considered directly connected, and PEs can exchange
OSPF routing information as they are using dedicated lines. This improves network
management and makes OSPF applications more effective.
As shown in Figure
21, and CE 22 belong to VPN 1. Assume that CE 11, CE 21, and CE 22 belong to the same
OSPF domain. PEs advertise VPN 1 routes in the following procedure:
a. PE 1 redistributes OSPF routes from CE 11 into BGP.
b. PE 1 advertises the VPN routes to PE 2 through BGP.
c. PE 2 redistributes the BGP VPN routes into OSPF and advertises them to CE 21 and CE
22.
Figure 80 Application of OSPF in VPN
With the standard BGP/OSPF interaction, PE 2 advertises the BGP VPN routes to CE 21 and
CE 22 through Type 5 LSAs (ASE LSAs). However, CE 11, CE 21, and CE 22 belong to the
same OSPF domain, and the route advertisement between them should use Type 3 LSAs
(inter-AS routes).
To solve the problem, the PE uses an extended BGP/OSPF interaction process called
BGP/OSPF interoperability to advertise routes from one site to another, differentiating the
routes from real AS-External routes. The process requires that extended BGP community
attributes carry the information for identifying the OSPF attributes.
80, PE 1 and PE 2 are connected through the MPLS backbone. CE 11, CE
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