OSPFCCNA 200-301

OSPF NSSA Area Not Converting Type 7 LSA to Type 5

Presenting Symptom

External routes redistributed into OSPF in an NSSA area are not appearing in the routing table of routers outside the NSSA area.

Network Context

The network consists of a small branch office configured as an OSPF NSSA area (Area 1) connected to the corporate backbone (Area 0) via an ABR. The branch router (R2) redistributes a static route into OSPF. Routers in Area 0 and other areas cannot see the external route. All routers run IOS 15.x.

Diagnostic Steps

Check OSPF neighbor relationships and area types

show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.1         1   FULL/DR         00:00:34    10.0.1.1        GigabitEthernet0/0

Ensure neighbors are FULL. If not, troubleshoot adjacency. Also verify area type on ABR with 'show ip ospf'.

Verify NSSA configuration on ABR and ASBR

show ip ospf interface <interface>

Internet Address 10.0.1.2/24, Area 1
  Process ID 1, Router ID 10.0.0.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 10.0.0.2, Interface address 10.0.1.2
  Backup Designated router (ID) 10.0.0.1, Interface address 10.0.1.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
  Area 1 is NSSA

Confirm that the interface is in NSSA area. If not, the area type is misconfigured.

Check for Type 7 LSAs on the ABR

show ip ospf database nssa-external

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

                Type-7 AS External Link States (Area 1)

  LS age: 100
  Options: (No TOS-capability, DC)
  LS Type: AS External (NSSA)
  Link State ID: 192.168.1.0 (External Network Number )
  Advertising Router: 10.0.0.2
  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: 0

If Type 7 LSAs are present on the ABR, the ABR should translate them to Type 5. If missing, the redistribution may not be happening.

Check for Type 5 LSAs in the backbone area

show ip ospf database external

            OSPF Router with ID (10.0.0.1) (Process ID 1)

                Type-5 AS External Link States

  LS age: 200
  Options: (No TOS-capability, DC)
  LS Type: AS External (Type 5)
  Link State ID: 192.168.1.0 (External Network Number )
  Advertising Router: 10.0.0.1
  LS Seq Number: 80000001
  Checksum: 0x5678
  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: 0

If Type 5 LSAs are missing, the ABR is not translating Type 7 to Type 5. Check the ABR's NSSA configuration for the 'no-redistribution' or 'default-information-originate' options that may affect translation.

Verify the ABR's NSSA translation status

show ip ospf

 Routing Process "ospf 1" with ID 10.0.0.1
  Start time: 00:00:00.000, Time elapsed: 1d00h
  Supports only single TOS(TOS0) routes
  Supports opaque LSA
  Supports Link-local Signaling (LLS)
  Supports area transit capability
  Router is not originating router-LSAs with maximum metric
  Initial SPF schedule delay 5000 msecs
  Minimum hold time between two consecutive SPFs 10000 msecs
  Maximum wait time between two consecutive SPFs 10000 msecs
  Incremental-SPF disabled
  Minimum LSA interval 5 secs
  Minimum LSA arrival 1000 msecs
  LSA group pacing timer 240 secs
  Interface flood pacing timer 33 msecs
  Retransmission pacing timer 66 msecs
  Number of external LSA 0. (0 checksum counter)
  Number of opaque AS LSA 0. (0 checksum counter)
  Number of DCbitless external and opaque AS LSA 0
  Number of DoNotAge external and opaque AS LSA 0
  Number of areas in this router is 2. 2 normal 0 stub 0 nssa
  Number of areas transit capable is 0
  External flood list length 0
  IETF NSF helper support enabled
  Cisco NSF helper support enabled
  Reference bandwidth unit is 100 mbps
    Area BACKBONE(0)
        Number of interfaces in this area is 1
        Area has no authentication
        SPF algorithm last executed 00:00:00.000 ago
        SPF algorithm executed 1 times
        Area ranges are
    Area 1
        Number of interfaces in this area is 1
        It is a NSSA area
        Perform type-7/type-5 LSA translation
        Area has no authentication
        SPF algorithm last executed 00:00:00.000 ago
        SPF algorithm executed 1 times
        Area ranges are

Look for 'Perform type-7/type-5 LSA translation' under the NSSA area. If missing, the ABR is not configured to translate. Also check 'Number of external LSA' — if 0, no Type 5 LSAs are being generated.

Root Cause

The ABR is not performing Type-7 to Type-5 LSA translation because the NSSA area was configured with the 'no-redistribution' option, or the ABR is not the elected translator. In this case, the ABR has the 'area 1 nssa no-redistribution' command, which prevents the ABR from injecting Type 7 LSAs into the backbone as Type 5.

Resolution

Remove the 'no-redistribution' keyword from the NSSA configuration on the ABR: R1(config)# router ospf 1 R1(config-router)# area 1 nssa If the ABR should not redistribute but still translate, ensure the 'no-redistribution' is not configured. Alternatively, if the ABR is not the translator, ensure it has a route to the forwarding address or use 'area 1 nssa translate type7 always' to force translation.

Verification

Run 'show ip ospf database external' on the ABR to see Type 5 LSAs. Also check the routing table on a backbone router: 'show ip route ospf' should now include the external route. Expected output: O E2 192.168.1.0/24 [110/20] via 10.0.1.2, 00:00:05, GigabitEthernet0/0

Prevention

1. Avoid using 'no-redistribution' on NSSA ABRs unless redistribution is intentionally blocked. 2. Use 'area <area-id> nssa translate type7 always' on the ABR to ensure translation even if the ABR is not the elected translator. 3. Verify NSSA translation with 'show ip ospf' after configuration.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why external routes are missing in an NSSA area. The exam tests understanding of NSSA LSA types (Type 7 vs Type 5) and the translation process. A candidate must know that the ABR translates Type 7 to Type 5 by default, and that 'no-redistribution' disables this translation.

Exam Tips

Remember that NSSA areas use Type 7 LSAs for external routes, and the ABR translates them to Type 5 for other areas.

The 'no-redistribution' keyword on the 'area nssa' command prevents the ABR from injecting Type 7 LSAs into the backbone as Type 5.

Use 'show ip ospf database nssa-external' and 'show ip ospf database external' to verify translation.

Commands Used in This Scenario

show ip ospf

Displays general information about OSPF routing process, including router ID, areas, and LSDB statistics, used to verify OSPF configuration and operational status.

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