Hi Vinod,
Good question, I never really noticed it before. The answer is in the RFC:
Inclusion of a forwarding address or external route tag in AS-external-LSAs is now optional. In addition, AS-external-LSAs can now reference another LSA, for inclusion of additional route attributes that are outside the scope of the OSPF protocol. For example, this reference could be used to attach BGP path attributes to external routes.
The external route tag is present in the AS-external-LSA if and only if the AS-external-LSA’s bit T is set.
Seems they decided to add a route tag by default for OSPFv3 external routes. We use these tags in redistribution:
Rene