# Prevention of Routing Loops Overview

PE routes disregard OSPF routes received from a CE router if the routes are advertised by:

- 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.

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.