• Link State protocol, knows entire topology of an area.
  • Most filtering occurs at area borders and autonomous system borders.

Filtering with prefix-list:

In the image above, R5 and R4 are in area 0, R2, R3 and R4 are in area 146. R5 is advertising the the loopback, which can be seen as an inter area route in R2.

To filter out this route a prefix list will need to be created on the ABR between 146 and 0, which is R4.


Above shows the prefix list is denying specifically the /32 host route. The next sequence down will permit all other prefixes. This list will then get applied to R4’s router OSPF process configuration.

Once the filter-list command is added, R5 no longer will see the subnet that was denied in the prefix list. The ending of the command ‘out’ is in place because we’re filtering a route coming from area 0, out to area 146.


  • Distribute-list is used with a standard access-list to deny traffic WITHIN an area or to a routing table of a specific host.
  • Distribute-list does not stop LSDBs, just stops a route from entering RIB post topology table.
  • Filter-lists are used with prefix-lists to stop an LSA summary (Type 3) from entering an area.
  • Filter-lists need to be completed on the ABR.

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