Lab Objective
Configure an OSPF ABR to filter a specific Type 3 LSA from propagating into a neighboring area while allowing other inter-area routes through normally, verifying selective route control at a granularity neither full summarization nor stub area configuration can achieve.
Lab Purpose
Area summarization, covered earlier in this series, aggregates multiple routes into one, and stub areas block entire categories of LSAs wholesale. Neither mechanism allows blocking one specific inter-area route while permitting others through unchanged — Type 3 LSA filtering fills this gap, offering ACL-like precision at the area boundary itself.
Lab Topology
R1 (Area 1) ---- R2 (ABR) ---- R3 (Area 0)
R1 has two LANs in Area 1:
192.168.70.0/24 (should reach Area 0 normally)
192.168.71.0/24 (should be BLOCKED from
reaching Area 0)Task 1: Verify Both Routes Currently Reach Area 0
Confirm R3 in Area 0 currently sees both of R1's LANs as Type 3 inter-area routes.
Task 2: Configure a Prefix List Matching the Route to Block
Create a prefix list specifically matching 192.168.71.0/24.
Task 3: Apply Type 3 LSA Filtering on the ABR
Apply the prefix list as an outbound Type 3 LSA filter on R2 toward Area 0.
Task 4: Verify the Filtered Route No Longer Reaches Area 0
Confirm R3 no longer sees 192.168.71.0/24, while 192.168.70.0/24 continues appearing normally.
Task 5: Verify R1's LAN Remains Reachable Locally Within Area 1
Confirm devices within Area 1 itself can still reach 192.168.71.0/24, since the filter only affects propagation into Area 0.
Solution and Verification
R3# show ip route ospf
O IA 192.168.70.0/24 [110/65] via 10.2.2.1
O IA 192.168.71.0/24 [110/65] via 10.2.2.1
-- Both routes currently reach Area 0 as
-- standard inter-area entriesR2(config)# ip prefix-list BLOCK-71 seq 5 permit 192.168.71.0/24
R2(config)# ip prefix-list BLOCK-71 seq 10 deny 0.0.0.0/0 le 32
-- Only the specific /24 is permitted here
-- for filtering purposes -- this prefix
-- list is inverted in usage compared to a
-- typical "permit what should pass" list,
-- since area filter-list's own permit/deny
-- keyword determines the actual actionR2(config)# router ospf 1
R2(config-router)# area 1 filter-list prefix BLOCK-71 out
-- "out" direction filters LSAs leaving
-- Area 1 toward other areas -- the prefix
-- list identifies which specific routes
-- this filter-list statement matches, and
-- since the filter-list is a "deny"-style
-- mechanism by nature, matched routes
-- (192.168.71.0/24) are blocked from
-- propagatingR3# show ip route ospf
O IA 192.168.70.0/24 [110/65] via 10.2.2.1
-- 192.168.71.0/24 has disappeared entirely
-- from Area 0's routing table, while
-- 192.168.70.0/24 continues propagating
-- completely unaffectedR1# ping 192.168.71.1
!!!!!
Success rate is 100 percent (5/5)
-- (pinged from another device within
-- Area 1 itself)
-- Local reachability within Area 1 is
-- completely unaffected -- the filter
-- only blocks the route's Type 3 LSA
-- specifically at the Area 1-to-Area 0
-- boundary, not the network's actual
-- existence or accessibility within its
-- own areaKey Takeaway
Type 3 LSA filtering with area filter-list operates with prefix-list precision at exactly the area boundary, blocking selected inter-area routes individually while leaving every other route through that same ABR completely unaffected — a level of granularity neither area range summarization nor the stub area family covered earlier in this series was designed to provide, making it the correct tool whenever specific routes need to be selectively withheld from a neighboring area for policy reasons rather than aggregated or blocked wholesale.