In OSPF VPN multiple instances, a link-state advertisement (LSA) tag is used to prevent routing loops between PEs.
In OSPF public network instances, the tag field does not prevent routing loops but is used to set the filtering conditions of routing policies.

The rules of setting the tags of ASE-LSAs and NSSA-LSAs are as follows:

In OSPF multiple instances, the route tag is set through the route-tag command.
In OSPF public networks, the route tag is set through the default tag command.
In either of the preceding two cases, you can run the import-route tag command to set the tag.

helpful info

nice post

good post

good doc

useful document, thanks