3.3.m [i] Route tagging, filtering
Users can define a route map to prevent OSPF routes from being added to the routing table. This filtering happens at the moment when OSPF is installing the route in the routing table. This feature has no effect on LSA flooding. In the route map, the user can match on any attribute of the OSPF route. That is, the route map could be based on the following match options:
match ip address
match ip next-hop
match ip route-source
match route-type match tag
OSPF external LSAs have a tag. The value of the tag is examined before the prefix is installed in the routing table . All OSPF external prefixes that have the tag value of 999 are filtered (prevented from being installed in the routing table). The permit statement with sequence number 20 has no match conditions, and there are no other route-map statements after sequence number 20, so all other conditions are permitted.
route-map tag-filter deny 10
match tag 999
route-map tag-filter permit 20
router ospf 1
network 172.16.2.1 0.0.0.255 area 0
distribute-list route-map tag-filter in
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2844-2848). . Kindle Edition.