Consider the following scenario, where routing loops could occur without using route tags:
Note the config snippets, below. In the first example, a route tag is set on all routes before being applied to the redistribute command. Therefore, all EIGRP routes will be redistributed into OSPF with a tag of 50.
Here we see evidence of the tag:
… and in the next snippet, the a deny-match statement references the tagged routes, with a second entry to permit all remaining prefixes. Then the route map is applied to ensure OSPF does not redistribute the routes learned from EIGRP back into it.
In a two-way mutual redistribution, to effectively mitigate risk of route loops, we should also be tagging the routes originally redistributed from OSPF to EIGRP and denying them from coming back.
Stuck-in-Active: Journal of an IT-Network Administrator 