OSPF Redistribution Not Bringing in External Routes
Presenting Symptom
External routes redistributed into OSPF are not appearing in the routing table of OSPF routers.
Network Context
A small enterprise campus network with two routers (R1 and R2) running OSPF in area 0. R1 is connected to an external network (EIGRP) and is configured to redistribute EIGRP routes into OSPF. R2 should learn these external routes but does not see them. Routers are Cisco IOS 15.x. The network has a single OSPF area with no special configurations.
Diagnostic Steps
Check OSPF neighbor adjacency
show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/DR 00:00:35 10.0.0.2 GigabitEthernet0/0
Ensure OSPF neighbors are in FULL state. If not, troubleshoot OSPF adjacency issues first.
Verify OSPF database for external routes
show ip ospf database external OSPF Router with ID (192.168.1.1) (Process ID 1)
Type-5 AS External Link States
LS age: 100
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.10.10.0 (External Network Number)
Advertising Router: 192.168.1.1
LS Seq Number: 80000001
Checksum: 0x1234
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0If external LSA (Type 5) is present, the redistribution is working but routes may not be installed. If missing, redistribution is not happening or LSA filtering is blocking.
Check routing table for external routes
show ip route ospfO E2 10.10.10.0/24 [110/20] via 10.0.0.2, 00:00:10, GigabitEthernet0/0
If external routes are not in the routing table, check for administrative distance issues or route filtering.
Verify redistribution configuration on the ASBR
show running-config | section router ospfrouter ospf 1 redistribute eigrp 100 subnets network 10.0.0.0 0.0.0.255 area 0
Ensure the redistribute command is present and includes the 'subnets' keyword. Missing 'subnets' causes only classful routes to be redistributed.
Check if default-information originate is needed
show ip route | include 0.0.0.0S* 0.0.0.0/0 [1/0] via 192.168.1.254
If redistributing a default route, ensure 'default-information originate' is configured under OSPF. Without it, the default route is not advertised.
Root Cause
The redistribute command under OSPF is missing the 'subnets' keyword. Without 'subnets', OSPF only redistributes classful networks (e.g., /8, /16, /24) and ignores subnetted routes. In this scenario, the external EIGRP routes are subnetted (e.g., /24), so they are not redistributed into OSPF.
Resolution
Verification
On R2, run 'show ip route ospf' and verify that the external routes now appear: R2# show ip route ospf 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O E2 10.10.10.0/24 [110/20] via 10.0.0.2, 00:00:15, GigabitEthernet0/0 Also verify the OSPF database on R2 shows the Type-5 LSA with the correct prefix.
Prevention
["Always include the 'subnets' keyword when redistributing into OSPF to ensure all subnetted routes are advertised.","Use route maps to control which routes are redistributed, rather than relying on default behavior.","Regularly audit redistribution configurations using 'show running-config | section router ospf'."]
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where OSPF external routes are missing. The exam tests understanding of the 'subnets' keyword in the redistribute command. Candidates may be asked to identify why redistributed routes are not appearing or to configure redistribution correctly. The key fact is that OSPF redistribution requires the 'subnets' keyword to include subnetted routes.
Exam Tips
Memorize that the 'subnets' keyword is required for OSPF redistribution of subnetted routes; without it, only classful routes are advertised.
In exam simulations, always check the redistribute command for the 'subnets' keyword when external routes are missing.
Remember that 'default-information originate' is needed to redistribute a default route into OSPF, separate from the redistribute command.
Commands Used in This Scenario
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