Troubleshooting OSPF and OSPF Configuration Verification

In this article, we are going to cover Open Shortest Path First (OSPF) configuration verification and troubleshooting. As a network engineer, you will likely encounter OSPF issues most of the time. OSPF is an open standard Interior Gateway Protocol (IGP) used for exchanging routing table information throughout a single Autonomous System (AS) in an IP network.

 

OSPF Configuration Verification

If you have access to the privileged mode of OSPF routers, you can use ‘show running-config’ to verify the configuration.

Router#show running-config 
Building configuration...
router ospf 1
 router-id 1.1.1.1
 network 1.1.1.0 0.0.0.3 area 0

 

If you only have access to user mode, then you can verify by using the ‘show ip protocols’ command. It will give you information like Router ID, the number of areas, network commands, and admin distance.

Router# sh ip protocols 
*** IP Routing is NSF aware ***

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:
    1.1.1.0 0.0.0.3 area 0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110)

 

We can also use ‘show ip ospf interface <brief>’ to verify interfaces that are actively participating in the OSPF process.

Router#  sh ip ospf interface 
GigabitEthernet0/0 is up, line protocol is up 
  Internet Address 1.1.1.1/30, Area 0, Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 1.1.1.2
  Backup Designated router (ID) 1.1.1.1, Interface address 1.1.1.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:02
  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 1 msec, maximum is 1 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)
Router#  sh ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/0        1     0               1.1.1.1/30         1     BDR   1/1

 

Troubleshooting OSPF

We can begin our troubleshooting by looking at the output of the ‘show ip ospf neighbor’ command.

Router#  show ip ospf neighbor  

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/DR         00:00:39    1.1.1.2         GigabitEthernet0/0

 

Here, we can verify if our router is forming an OSPF neighbor, the interface, neighbor’s IP address, and ID.
The normal working states are:

OSPF Working StateDefinition
FULL/ –The “-” instead of letters in the neighbor state indicates that the link does not use a DR/BDR (Designated Router/Backup Designated Router).
FULL/DRThe OSPF neighbor state is full, and the neighbor router is the DR.
FULL/BDRThe OSPF neighbor state is full, and the neighbor router is the backup DR (BDR).
FULL/DROTHERThe OSPF neighbor state is full, and the neighbor router is neither the DR nor the BDR. (Because the status is FULL, it also suggests that the local router is a DR or BDR.)
2WAY/DROTHERThe OSPF neighbor state is two-way, and the neighbor is a DROther router who is neither the DR nor the BDR. (It also suggests that the local router is a DROther router, because else the state would be FULL.)

 

Another helpful command is ‘show ip ospf database’, which lists all OSPF routers in the same area.

Router# 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         18          0x80000003 0x00F926 1
2.2.2.2         2.2.2.2         2           0x80000003 0x00BB5B 1

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.2         2.2.2.2         2           0x80000002 0x008D95

 

The ‘show ip ospf’ command will show Router ID, reference bandwidth, number of interfaces, and authentication.

Router#show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
 Reference bandwidth unit is 100 mbps
    Area BACKBONE(0)
        Number of interfaces in this area is 1
        Area has no authentication

 

Listed below are some of the useful debug commands issued at the privilege mode that we can use in troubleshooting and debugging the OSPF implementation in our network:
debug ip ospf packet – shows hello packets that are being sent and received on the router
debug ip ospf – shows the mismatch in hello parameters.
debug ip ospf adj – shows DR and BDR elections on a broadcast or non-broadcast multi-access network


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