• 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 192.168.5.1/32, 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.

Configuration:

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.

NOTE:

  • 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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