Is a tag value unique to the local routing process or it is unique for the entire network? In another word, if I use a tag value to tag a route, can I use the same tag value on another router for a different route tagging? It seems like OSPF LSA packet doesnt have a tag field.
think of it as similar to BGP community a route tag is a way to provide a color to a group of routes.
The routes travel with their route tags in all the OSPF domain (they are LSA type 5 they cannot be filtered they can be blocked at any type of stub area boundary).
Another ASBR has to mark routes that is injecting into OSPF.
the typical route-map with route tags has the logic
route-map asbr-tags deny 10
match tag xxxx
route-map asbr-tags permit 20
set tag xxxx
in this way you don't send back the routes you have learned before from out of OSPF.
if you use the same route-tag the risk is that you can filter some routes that actually haven't come from this ASBR boundary but they had travelled in the domain.
This can cause ip reachability problems.
For this reason is better to use different route tag values in different redistribution points.
However, if you have two ASBR nodes that redistribute from the same EIGRP domain into OSPF in this case you use the same route-tag to avoid to send back info learned on ASBR1 via ASBR2 to EIGRP domain.
So the suggestion is to use different route tags for different external routing domains.
Hope to help