ospf troubleshooting questions

OSPF is one of the more complicated routing protocols, and it can be pretty threatening.  Therefore, troubleshooting OSPF and verifying the configurations are very important. There are several ways of verifying and troubleshooting OSPF:-

Verify OSPF Neighbors

Figure 1 shows the reference topology for this lesson. We can show and verify the OSPF neighbor adjacency using the “show ip ospf neighbor” command. If the router is not showing the state of Full, the routers have not formed an OSPF adjacency.

The causes of routers do not establish adjacency, and LSAs are not exchanged between routers. Incomplete LSDBs can also cause inaccurate SPF trees and routing tables if the route to the destination does not exist or may not be the most optimum path for the destination. Figure 2 displays the Router1 neighbor adjacency.

The parameters shown in this command are as follows:-
Neighbor ID – The unique ID of the neighboring router.
Pri – The OSPF priority of the interface. This value is used in the DR and BDR elections.
State – This is the OSPF state of the interfaces. FULL state means that the router and its neighbor have identical OSPF LSDBs. The dash indicates that no DR or BDR is required because of the network type. On multi-access networks, such as Ethernet, two adjacent routers may have their states displayed as 2WAY. The DR show the adjacent router connected to this interface is DR.

Dead Time – The remaining time to receive an OSPF Hello packet from the neighbor before declaring the neighbor down. This value is reset when the interface receives a Hello packet.
Address – The IP address of the neighbor router interface to which this router is directly connected.
Interface – The router interface on which it has formed an adjacency with the neighbor router.

We can also verify Router2, Router3, and Router4 using the “show ip ospf neighbor” command and get help troubleshooting OSPF on these routers. If the adjacency did not form any router. Check the following:-

If the subnet masks do not match.
OSPF Hello and Dead timers do not match.
OSPF Network Types do not match.
There is a missing “network” command.
Incorrect OSPF “network” command.

Verify OSPF Protocol Settings

We can verify the OSPF protocol settings using the “show ip protocols” to verify important OSPF configuration information. The command displays the OSPF process ID, the router ID, the advertised network, the neighbors the router is receiving updates from, and also the default administrative distance. The figure-3 Illustrates the output of the “show ip protocols” command. Use the command on other routers to verify the above information.

Verify OSPF Process Information

We can also verify the OSPF process ID and router ID using the “show ip ospf” command, as shown in Figure 4. This command displays the OSPF area information and the last time the SPF algorithm was calculated.

Verify OSPF Interface Settings

The command “show ip ospf interface” can display the interface settings. With this command, we can display and check a detailed list for every OSPF-enabled interface. The command of whether the network statements are correctly composed or not.

We can only get the summary using the command “show ip ospf interface brief”. Figure 5 displays detailed information about this command. We can also specify the interface name with this command to display the information of the particular interface, for example “show ip ospf interface serial 0/3/1.

Verify OSPF Database

This command “show ip ospf database” displays the information on the number of routers in the network or internetwork plus the ID of the neighbouring router.

The figure 6 illustrates the output of this command. The ADV router is the advertising router. The checksum link count might display different numbers depending on the routing device.

The Debugging of OSPF

This is an essential command for troubleshooting OSPF. We can use this command in the following ways:
Debug ip ospf event: This command displays hello packets that are sent and received.
Debug ip OSPF adj: shows DR and DBR elections on broadcast and non-broadcast multi-access networks.