Loop Prevention : OSPF Down Bit and Domain Tag

Loop Prevention in MPLS VPN Domain using OSPF

Down Bit


Routing loops can occur in the MPLS VPN environment when customer edge routers are dual-homed to the service provider network. MPLS VPN network implementing OSPF PE-CE routing for Customer A VPN-A sites, Site 1 and Site 2. Site 2 is in OSPF Area 2 and has multiple connections to the provider backbone.

The routing loop can be prevented by the use of the OSPF down bit, which is part of the options field in the OSPF header. The LSA header with the option field

Tag

The down bit helps prevent routing loops between MP-BGP and OSPF, but not when external routes are announced, such as when redistribution between multiple OSPF domains or when external routes are injected in an area that is dual-homed to the provider network. The PE router redistributes an OSPF route from a different OSPF domain into an OSPF domain as an external route. The down bit is not set because LSA Type 5 does not support the down bit. The redistributed route is propagated across the OSPF domain.


The routing loops introduced by route redistribution between OSPF domains can be solved with the help of the tag field, using standard BGP-OSPF redistribution rules. A non-OSPF route is redistributed as an external OSPF route by a PE router. By default, the tag field is set to the BGP-AS number. The redistributed route is propagated across the OSPF domain without the down bit but with the tag field set. When the route is redistributed into another OSPF domain, the tag field is propagated. Another PE router receives the external OSPF route and filters the route based on the tag field. The tag field matches the AS number so the route is not redistributed into MP-BGP