The value of tmnxOspfVpnTag specifies the route tag for an OSPF VPN
on a PE router. This field is set in the tag field of the OSPF external
LSAs generated by the PE. This is mainly used to prevent routing loops.
This object applies to VPRN instances of OSPF only. An attempt to
modify the value of this object will result in an 'inconsistentValue'
error when the vRtrID index for this row entry is not a VPRN instance.