3.3.m Implement and troubleshoot loop prevention mechanisms

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 interface

match ip address

match ip next-hop

match ip route-source

match metric

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 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.