3Com MSR 50 Series Configuration Manual page 1478

1478 C 79: MPLS L3VPN C
HAPTER ONFIGURATION
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 Figure 420, both site 1 and site 3 belong to
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.