HP HSR6800 Configuration Manual page 264

Each OSPF domain must have a configurable domain ID. As a best practice, configure the
same domain ID or adopt the default ID for all OSPF processes of the same VPN, so the system
can know that all VPN routes with the same domain ID are from the same VPN.
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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 learned from MPLS/BGP to the VPN site through LSAs, the
LSAs can 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 learned 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
site 2 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.
Figure 81 Network diagram for sham link
To resolve the problem, you can establish a sham link between the two PEs so that the routes
between them over the MPLS VPN backbone become an intra-area route.
The sham link acts as an intra-area point-to-point link and is advertised through the Type 1 LSA. You
can select a route between the sham link and backdoor link by adjusting the metric.
The sham link is considered the link between the two VPN instances with one endpoint address in
each VPN instance. The endpoint address is a loopback interface address with a 32-bit mask in the
VPN address space on the PE. Different sham links of the same OSPF process can share an
endpoint address, but that of different OSPF processes cannot.
BGP advertises the endpoint addresses of sham links as VPN-IPv4 addresses. A route across the
sham link cannot be redistributed into BGP as a VPN-IPv4 route.
A sham link can be configured in any area. You must configure it manually. In addition, the local VPN
instance must have a route to the destination of the sham link.
NOTE:
When configuring an OSPF sham link, redistribute OSPF VPN routes to BGP, but do not redistribute
BGP routes to OSPF to avoid route loops.
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Figure 81, both site 1 and