Genel olarak PE-CE OSPF kullanılması durumlarında, döngüyü engellemek için, Down bit kullanımının desteklenmediği durumlarda kullanılır. Kısaca PE OSPF içerisine route redistribit yapıyor ise (BGP’den duduğu routeları) bunları route-tag ile set eder. Eğer kendi route-tag ile aynı değerde routeları OSPF’den alır ise ne kullanır nede tekrar IBGP ile anons eder. route-tag genel olarak AS numarasından hesaplanır. Bu şekilde her iki PE’de de aynı route-tag kullanılması sağlanır.
RFC’ye göre :
If a particular VRF in a PE is associated with an instance of OSPF, then by default it MUST be configured with a special OSPF route tag value, which we call the VPN Route Tag. By default, this route tag MUST be included in the Type 5 LSAs that the PE originates (as the result of receiving a BGP-distributed VPN-IPv4 route, see Section 4.2.8) and sends to any of the attached CEs.
The configuration and inclusion of the VPN Route Tag is required for backward compatibility with deployed implementations that do not set the DN bit in type 5 LSAs. The inclusion of the VPN Route Tag may be disabled by configuration if it has been determined that it is no longer needed for backward compatibility.
The value of the VPN Route Tag is arbitrary but must be distinct from any OSPF Route Tag being used within the OSPF domain. Its value MUST therefore be configurable. If the Autonomous System number of the VPN backbone is two bytes long, the default value SHOULD be an automatically computed tag based on that Autonomous System number
If the Autonomous System number is four bytes long, then a Route Tag value MUST be configured, and it MUST be distinct from any Route Tag used within the VPN itself.
If a PE router needs to use OSPF to distribute to a CE router a route that comes from a site outside the CE router’s OSPF domain, the PE router SHOULD present itself to the CE router as an Autonomous System Border Router (ASBR) and SHOULD report such routes as AS-external routes. That is, these PE routers originate Type 5 LSAs reporting the extra-domain routes as AS-external routes. Each such Type 5 LSA MUST contain an OSPF route tag whose value is that of the VPN Route Tag. This tag identifies the route as having come from a PE router. The VPN Route Tag MUST be used to ensure that a Type 5 LSA originated by a PE router is not redistributed through the OSPF area to another PE router.