• Tagging provides a way to mark common or similar routes to manipulate later.
• In redistribution scenarios with mutual redistribution on two different routers, any routes that gets redistributed from one route process to another are tagged.
  • When the other router sees those tags on the route, that route to keep from adding non-optimal routes to its routing table.
• Tags can also be used to do other manipulation such as setting higher metrics or changing ADs.

OSPF

EIGRP allows you to tag external routes.  That is, any route redistributed into EIGRP can be tagged with a numeric descriptor from 0 to 4294967295.