3Com S7906E Configuration Manual page 1535

Finally, PE 2 redistributes the BGP VPN routes into OSPF and advertises them to CE 21 and CE
22.
Figure 1-16 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-provider routes).
To solve the above problems, 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.
It is required that each OSPF domain has a configurable domain ID. It is recommended to configure for
all OSPF instances in the network related to each VPN instance the same domain ID, or adopt the
default ID. Thus, the system can know that all VPN routes with the same domain ID are from the same
VPN instance.
3) Routing loop detection
If OSPF runs between CEs and PEs and a VPN site is connected to multiple PEs, when a PE advertises
the BGP VPN routes learnt from MPLS/BGP to the VPN site through LSAs, the LSAs may be received
by another PE, resulting in a routing loop.
To avoid routing loops, when creating Type 3 LSAs, the PE always sets the flag bit DN for BGP VPN
routes learnt from MPLS/BGP, regardless of whether the PE and the CEs are connected through the
OSPF backbone. When performing route calculation, the OSPF process of the PE ignores the Type 3
LSAs whose DN bit is set.
If the PE needs to advertise to a CE the routes from other OSPF domains, it must indicate that it is the
ASBR, and advertise the routes using Type 5 LSAs.
Sham link
Generally, BGP peers carry routing information on the MPLS VPN backbone through the BGP extended
community attributes. The OSPF that runs on the remote PE can use the information to create Type 3
summary LSAs to be transmitted to the CEs. As shown in
VPN 1 and OSPF area1. They are connected to different PEs, PE 1 and PE 2. There is an intra-area
OSPF link called backdoor link between them. In this case, the route connecting the two sites through
PEs is an inter-area route. It is not preferred by OSPF because its preference is lower than that of the
intra-area route across the backdoor link.
Figure 1-17, both site 1 and site 2 belong to
1-20