OSPF LSDB Incomplete — Missing LSA Types
Presenting Symptom
The OSPF routing table is missing some routes, and the OSPF database shows incomplete LSA types (e.g., Type 3 or Type 5 LSAs missing) on some routers.
Network Context
A small enterprise campus network with three routers (R1, R2, R3) running OSPF in a single area (Area 0). R1 and R2 are ABRs connecting to other areas, and R3 is an internal router. All routers run IOS 15.x. The network uses OSPF for internal routing, and some routers are configured with multiple areas. The issue is observed after a recent configuration change on R2.
Diagnostic Steps
Check OSPF neighbor relationships
show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 FULL/DR 00:00:35 10.0.1.2 GigabitEthernet0/0 10.0.0.3 1 FULL/BDR 00:00:38 10.0.2.3 GigabitEthernet0/1
All OSPF neighbors should be in FULL state. If any neighbor is in EXSTART or EXCHANGE, there may be a database synchronization issue. If neighbors are missing, check connectivity and OSPF configuration.
Examine the OSPF database for missing LSA types
show ip ospf database OSPF Router with ID (10.0.0.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.0.1 10.0.0.1 100 0x80000004 0x00A1B2 2
10.0.0.2 10.0.0.2 200 0x80000003 0x00C3D4 3
10.0.0.3 10.0.0.3 150 0x80000002 0x00E5F6 1
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.1.2 10.0.0.2 180 0x80000001 0x00AABB
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.1.0 10.0.0.2 50 0x80000001 0x001122
192.168.2.0 10.0.0.2 60 0x80000001 0x003344
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
0.0.0.0 10.0.0.1 300 0x80000001 0x005566 10Look for missing LSA types. For example, if Type 3 (Summary) LSAs are missing, inter-area routes may not be propagated. If Type 5 (External) LSAs are missing, redistributed routes are absent. Compare with a known-good router's database.
Check OSPF interface configuration for area assignment
show ip ospf interfaceGigabitEthernet0/0 is up, line protocol is up
Internet Address 10.0.1.1/24, Area 0
Process ID 1, Router ID 10.0.0.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 10.0.0.1, Interface address 10.0.1.1
Backup Designated router (ID) 10.0.0.2, Interface address 10.0.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 10.0.0.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)Verify that each interface is in the correct OSPF area. If an interface is in the wrong area, it can cause LSA type mismatches. Also check that the network type is consistent (e.g., broadcast vs point-to-point).
Check OSPF redistribution and route filtering
show run | section router ospfrouter ospf 1 router-id 10.0.0.1 redistribute static subnets network 10.0.0.0 0.0.0.255 area 0 network 192.168.1.0 0.0.0.255 area 1 distribute-list prefix FILTER in
Look for redistribution statements and distribute-lists. A missing redistribute command or a distribute-list that filters out LSAs can cause incomplete databases. Also check for area filtering (area 0 filter-list prefix ...).
Root Cause
On R2, a distribute-list prefix FILTER in was applied under router ospf, which filters incoming Type 3 LSAs from Area 0, preventing them from being installed in the OSPF database. This caused R2 to not have the summary LSAs for networks in other areas, and thus those routes are missing from the routing table.
Resolution
Verification
Run 'show ip ospf database' on R2 to confirm that Type 3 LSAs now appear: R2# show ip ospf database ... Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 192.168.1.0 10.0.0.1 10 0x80000002 0x001122 192.168.2.0 10.0.0.1 15 0x80000002 0x003344 ... Also verify that the missing routes appear in the routing table: R2# show ip route ospf ... O IA 192.168.1.0/24 [110/2] via 10.0.1.1, 00:00:10, GigabitEthernet0/0 O IA 192.168.2.0/24 [110/2] via 10.0.1.1, 00:00:10, GigabitEthernet0/0 ...
Prevention
1. Avoid using distribute-lists under OSPF unless absolutely necessary; use area filtering (area filter-list) for more granular control. 2. Always verify OSPF database completeness after any configuration change. 3. Use OSPF route summarization on ABRs instead of filtering to reduce LSA flooding.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests understanding of OSPF LSA types (Type 1-5) and how filtering affects the LSDB. Expect multiple-choice questions asking which LSA type is missing given a symptom, or drag-and-drop steps to troubleshoot missing OSPF routes. Key fact: distribute-lists filter routes but not LSAs in the database; they affect the routing table only.
Exam Tips
Remember that distribute-lists under OSPF filter routes from the routing table, not LSAs from the database; the LSDB may still contain the LSAs.
Be able to identify missing LSA types: Type 1 (Router), Type 2 (Network), Type 3 (Summary), Type 4 (ASBR Summary), Type 5 (External).
Know that 'show ip ospf database' shows all LSAs, while 'show ip route ospf' shows installed routes; a mismatch indicates filtering.
Commands Used in This Scenario
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