OSPF Route Filtering

OSPF ROUTE FILTERING

OSPF is a Link State IGP

• Filter will apply very specific points within the OSPF topology
•  It should filter mostly at area border router
•  Some cases, can apply to routing table itself 

 Filter between Areas

• Filtering within Summarization
• When generating an area range, can set "do not advertise flag "
• LSA Type-3 Filtering
• Apply to ABR, taking Type 1 and 2 LSAs, summarizing the topology info 
• And then select permit or deny them... "area filter list"
• NSSA ABR External Prefix filtering
• Do not set to P-bit, so will not able to translate Type7 to 5 
• Multiple ABRs of the NSSA connecting to Area 0, need to check translator election
• Based on Router ID, or configure "translate always" manual set for election
• Transit prefix suppression
•  Especially for MPLS vpn core, can suppress the transit prefixes in type 1 LSAs

Filter with OSPF RIB

• Distribute-list with ACL
• Distribute-list with Route-Map
• Administrative Distance 

Useful Links

1. Filtering With Route Map and Distribute List
2. ABR Type 3 LSA Filtering 
3. Forwarding Address Suppression in Translated Type-5 LSAs 
4. Inbound Filtering Using Route Maps With A Distribute List