Figure 112: Ospf Topology With Backdoor Link - Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual

JUNOSe 11.0.x BGP and MPLS Configuration Guide
Preventing Routing Loops
PE routes disregard OSPF routes received from a CE router if the routes are advertised
by:
When the destination PE router originates a type 3 LSA learned from BGP to a CE
router, the PE router sets the most-significant bit in the LSA options field to identify
the LSA as being generated from a PE router. Doing this prevents the LSA from being
passed back to the BGP/MPLS VPN through a different PE router.
When the destination PE router originates a type 5 LSA learned from BGP to a CE
router, the PE router replaces the external route tag in the LSA with the VPN route
tag. You configure the VPN route tag for the OPSF VRF on the PE router with the
domain-tag command. The value of a VPN route tag must be unique within an OSPF
domain, so that the same external route is not propagated back to the BGP/MPLS
VPN backbone through another PE-CE link.
Using Remote Neighbors to Configure OSPF Sham Links
When you employ OSPF as the PE-CE routing protocol in a BGP/MPLS VPN and also
configure OSPF backdoor links between VPN sites outside the backbone, the backdoor
links are always preferred over the backbone paths between the VPN links. OSPF
sham links prevent this problem, and you can implement them with OSPF remote
neighbors. Consider the topology shown in Figure 112 on page 476.

Figure 112: OSPF Topology with Backdoor Link

The PE routers are each running a separate logical OSPF instance for each VRF. Each
of these OSPF instances has adjacencies with their directly connected CE routers
and exchanges LSAs with those CE routers. The OSPF routes that are learned from
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OSPF and BGP/MPLS VPNs
A type 3 LSA with the most-significant bit set in the LSA options field.
A type 5 LSA that has a tag value equal to the VPN route tag associated with the
OSPF VRF on that PE router.