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OSPFCCNA 200-301

OSPF Route Not Appearing in Routing Table

Presenting Symptom

A route for a specific network is missing from the routing table on a router running OSPF.

Network Context

The network consists of three routers (R1, R2, R3) in a small branch office, all running OSPF in area 0. R1 and R2 are connected via a serial link, R2 and R3 via Ethernet. All routers are Cisco IOS 15.x. The missing route is for a loopback interface on R3 that should be advertised into OSPF.

Diagnostic Steps

1

Check OSPF neighbor relationships

show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2          1    FULL/DR         00:00:34    10.1.1.2        GigabitEthernet0/0
3.3.3.3          1    FULL/BDR        00:00:38    10.2.2.3        GigabitEthernet0/1

If neighbors are not FULL, OSPF adjacency is not established. Check for mismatched parameters (area, network type, authentication). If neighbors are FULL, proceed to next step.

2

Verify OSPF database for the missing route

show ip ospf database
       OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         123         0x80000002 0x00A1B2 1
2.2.2.2         2.2.2.2         456         0x80000001 0x00C3D4 2
3.3.3.3         3.3.3.3         789         0x80000003 0x00E5F6 2

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.2.2.2        2.2.2.2         234         0x80000001 0x00AABB

Check if the missing route's network appears in the database. If it does not, the route is not being originated. If it appears, the issue is with route calculation or installation.

3

Check if the route is in the OSPF routing table

show ip route ospf
O    10.1.1.0/24 [110/2] via 10.1.1.2, 00:01:23, GigabitEthernet0/0
O    10.2.2.0/24 [110/2] via 10.2.2.3, 00:00:45, GigabitEthernet0/1

If the missing route is not listed, OSPF is not installing it. Check for administrative distance issues, route filtering, or mismatched subnet masks.

4

Verify OSPF interface configuration on the originating router

show ip ospf interface GigabitEthernet0/1
GigabitEthernet0/1 is up, line protocol is up
  Internet Address 10.2.2.3/24, Area 0
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 10.2.2.2
  Backup Designated router (ID) 3.3.3.3, Interface address 10.2.2.3
  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 2.2.2.2  (Designated Router)
  Suppress hello for 0 neighbor(s)

Check that the interface is OSPF-enabled and in the correct area. If the interface is not listed, OSPF is not enabled on that interface. Also verify the network type and cost.

5

Check for route filtering or redistribution issues

show ip protocols
Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 1.1.1.1
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    10.1.1.0/24
    10.2.2.0/24
  Routing Information Sources:
    Gateway         Distance      Last Update
    2.2.2.2              110      00:01:23
    3.3.3.3              110      00:00:45
  Distance: (default is 110)

Check if any distribute-lists or route-maps are applied. Also verify that the network statement includes the correct subnet. If the missing route is from a different protocol, check redistribution.

Root Cause

The missing route is not being advertised because the network statement on R3 does not include the loopback interface's subnet. The loopback interface (192.168.1.1/32) is not covered by any 'network' command under OSPF process 1 on R3.

Resolution

On R3, enter global configuration mode and add the network statement for the loopback interface: R3# configure terminal R3(config)# router ospf 1 R3(config-router)# network 192.168.1.1 0.0.0.0 area 0 R3(config-router)# end This command advertises the loopback interface (using a wildcard mask of 0.0.0.0 for a /32) into OSPF area 0.

Verification

On R1, run 'show ip route ospf' and verify the route appears: R1# show ip route ospf O 192.168.1.1/32 [110/3] via 10.1.1.2, 00:00:05, GigabitEthernet0/0 Also verify the OSPF database on R1 includes the new LSA: R1# show ip ospf database router 3.3.3.3 (should show the loopback link)

Prevention

1. Use the 'network' command with the exact subnet and wildcard mask for loopback interfaces, or use 'ip ospf 1 area 0' under the interface configuration mode for more granular control. 2. Regularly audit OSPF configurations to ensure all desired networks are included in the OSPF process. 3. Implement a change management process that requires verification of routing tables after any OSPF configuration changes.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a route is missing. Candidates must know that OSPF only advertises networks explicitly configured under the OSPF process. The exam may present a drag-and-drop to match symptoms with causes, or a multiple-choice question asking why a route is missing. Key fact: OSPF network statements use wildcard masks, and loopbacks are typically advertised as /32 host routes.

Exam Tips

1.

Remember that OSPF network statements use wildcard masks (inverse of subnet mask). For a /32, use 0.0.0.0.

2.

If a route is missing, first check OSPF neighbors, then the database, then the routing table. The issue is often a missing network statement or a mismatched area.

3.

Know the difference between 'network' under router ospf and 'ip ospf' under interface. The interface method is more precise and recommended for loopbacks.

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 interface

Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.

show ip ospf neighbor

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

show ip protocols

Displays the current state of all IP routing protocols running on the router, including timers, filters, and network advertisements.

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