The VPN route tag used for loop detection as part of OSPF PE-CE protocol function. This field is only used if ospfPmEntVpnPeCeSupport is true. If this field is set to zero then VPN route tag function is disabled. VPN route tag function is specified in RFC 4577, and VPN route tag configuration is described in sections 4.2.5.1 and 4.2.5.2. In particular RFC 4577 sectios 4.2.5.1 and 4.2.5.2 state the following. - If OSPF PE-CE protocol function is enabled then by default a VPN route tag MUST be configured. - If the Autonomous System number of the VPN backbone is two bytes long, the VPN route tag SHOULD be set to 0xD000nnnn where nnnn is the AS number of the VPN backbone. - When it is no longer necessary to use the VPN Route Tag in a particular deployment, its use (both sending and receiving) may be disabled by configuration. If ospfPmEntVpnRouteTag is non-zero then it overrides any OSPF tag supplied with a redistributed route. In particular, it overrides any OSPF tag specified by rtmRedistNwOspfTag in the rtmRedistTable MIB for this OSPF instance.