OSPF Filtering

Summary of OSPF filtering options

• Prefix suppression 

- can be under interface level or process level

Note: When configured under process level, it will not suppress prefixes associated with Loopback interfaces and Passive interfaces. Under interface level, it will not suppressed the secondary ip address network 

• Filter list (on the ABR)

- area x filter-list prefix "name" [in | out]

• Area range (ABR) and Summary address (ASBR) with not-advertise

- area x range "prefix" not-advertise
- summary-address "prefix" not-advertise

• Distribute list (via acl or prefix-list)

•  Cost and AD

Note: the last two ways are filtering prefixes from/into Routing table, but not from the LSDB. This sometimes can lead to blackholes in the topology.