OSPFCCNA 200-301

OSPF Default Route Not Propagated to Other Areas

Presenting Symptom

Hosts in a non-zero OSPF area cannot reach the internet, even though the Area Border Router (ABR) has a default route to the internet.

Network Context

The network consists of a three-router OSPF topology: R1 (ASBR) connected to the internet and to R2 (ABR) in area 0; R2 connects to R3 in area 1. All routers run IOS 15.x. The ASBR has a default route via an external interface, but hosts in area 1 cannot ping external IPs.

Diagnostic Steps

Check OSPF neighbor relationships on the ABR

show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.3         1   FULL/DR         00:00:35    10.0.1.2        GigabitEthernet0/1
10.0.0.1         1   FULL/DR         00:00:38    10.0.0.1        GigabitEthernet0/0

Verify that the ABR has FULL adjacency with both the ASBR (area 0) and the internal router (area 1). If neighbors are missing, troubleshoot OSPF adjacency issues first.

Verify the ASBR's default route and redistribution

show ip route ospf | include 0.0.0.0/0

O*E2 0.0.0.0/0 [110/1] via 10.0.0.2, 00:10:15, GigabitEthernet0/0

On the ASBR, confirm a default route is present and redistributed into OSPF. If missing, check the default route configuration and redistribution. The 'O*E2' indicates an OSPF external route.

show ip ospf database external

OSPF Router with ID (10.0.0.1) (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: 0.0.0.0 (External Network Number)
  Advertising Router: 10.0.0.1
  LS Seq Number: 80000001
  Checksum: 0x1234
  Length: 36
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 0

On the ABR, check if the Type 5 LSA for the default route is present. If it is missing, the ASBR may not be generating it or the ABR is not receiving it. If present, the issue is likely that the ABR is not propagating it into the non-zero area.

show ip ospf database summary (on ABR)

OSPF Router with ID (10.0.0.2) (Process ID 1)

                Type-4 ASBR Summary Link States

  LS age: 50
  Options: (No TOS-capability, DC)
  LS Type: ASBR Summary Link
  Link State ID: 10.0.0.1 (ASBR Router ID)
  Advertising Router: 10.0.0.2
  LS Seq Number: 80000001
  Checksum: 0xABCD
  Length: 28
  Network Mask: /0
        TOS: 0
        Metric: 1

Check for Type 4 LSA (ASBR Summary) on the ABR. If missing, the ABR is not generating the Type 4 LSA for the ASBR, which is required for routers in other areas to reach the ASBR. This is a common root cause.

show run | section router ospf

router ospf 1
 network 10.0.0.0 0.0.0.255 area 0
 network 10.0.1.0 0.0.0.255 area 1
 default-information originate

On the ASBR, verify that 'default-information originate' is configured. If it is missing, the ASBR will not inject the default route into OSPF. Also check that the ABR does not have any distribute-list filtering Type 5 LSAs.

Root Cause

The ABR (R2) is not generating a Type 4 ASBR Summary LSA for the ASBR (R1). This occurs when the ABR does not have a route to the ASBR's router ID via area 0, or when the ABR's OSPF process is misconfigured (e.g., missing network statement for the link to the ASBR). Without the Type 4 LSA, routers in area 1 do not know how to reach the ASBR and therefore cannot use the default route.

Resolution

On the ABR (R2), ensure that the interface connecting to the ASBR is correctly included in area 0. If missing, add the network statement. Also verify that the ABR has a route to the ASBR's router ID (e.g., via a loopback). If the ABR's router ID is not reachable, configure a loopback interface with an IP in area 0 and set it as the router ID. Example configuration on R2: interface Loopback0 ip address 2.2.2.2 255.255.255.255 ! router ospf 1 router-id 2.2.2.2 network 10.0.0.0 0.0.0.255 area 0 network 10.0.1.0 0.0.0.255 area 1 If the issue persists, ensure that the ASBR's router ID is reachable from the ABR (e.g., ping 1.1.1.1 from R2).

Verification

On a router in area 1 (R3), run: show ip route ospf | include 0.0.0.0/0 Expected output: O*E2 0.0.0.0/0 [110/1] via 10.0.1.1, 00:00:10, GigabitEthernet0/0 Also run: show ip ospf database external Expected output: Type-5 LSA for 0.0.0.0 with Advertising Router 10.0.0.1 And: show ip ospf database asbr-summary Expected output: Type-4 LSA for ASBR 10.0.0.1 advertised by 10.0.0.2

Prevention

1. Always configure a loopback interface on ABRs and ASBRs with an IP in area 0 to ensure stable router ID reachability. 2. Use 'default-information originate always' on the ASBR to ensure the default route is injected even if the ASBR loses its own default. 3. Verify OSPF database consistency across areas using 'show ip ospf database' on ABRs.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a default route is not being propagated to non-zero areas. The exam tests understanding of OSPF LSA types (Type 4 and Type 5) and the role of ABRs. Candidates must know that Type 4 LSAs are required for inter-area external route reachability.

Exam Tips

Remember that Type 4 LSAs are generated by ABRs to advertise the location of ASBRs to other areas.

If a default route is not appearing in a non-zero area, check for missing Type 4 LSA using 'show ip ospf database asbr-summary'.

The command 'default-information originate' must be configured on the ASBR; without it, no default route is injected into OSPF.

Commands Used in This Scenario

show ip ospf database

Displays the OSPF link-state database (LSDB) to verify OSPF adjacencies, check for missing or corrupt LSAs, and troubleshoot OSPF routing issues.

show ip ospf neighbor

Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.

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