Troubleshooting Common Cisco OSPF Issues

OSPF Troubleshooting

For those just getting into network routing protocols, it is hard enough to initially understand how all of them work. But what happens when they are not working in the way that you think they should? This article takes a look at some of the most common configuration mistakes that can often become problems for students just getting started with Open Shortest Path First (OSPF).

OSPF is a standard based network routing protocol that is typically used for routing traffic within an internal network. These types of protocols are referred to as Interior Gateway Protocols (IGP). While the term internal may make it sound like the range of OSPF is limited to a small network, OSPF was built for exactly the opposite. OSPF has the ability to scale from a small network with a few routers to the largest international corporations spanning thousands of routers wide. This is one of the reasons that it is very popular. It however does have its drawbacks, mainly that it can be quite complex to configure.

OSPF implements its scalability by implementing areas. These areas are then allocated depending on the physical and technical details of a specific network (are all the routers in the same building, same floor, same city…). One central area, referred to as the backbone or area 0, acts as a central hub between all other areas. Typically all traffic that must go from a device in one area to a device in another area must, at some point, pass through the backbone.

OSPF routers maintain relationships with other directly connected routers, referred to as neighbors, to obtain a complete list of the reachable networks. In this exchange, the routers will tell each other about the routes that they know about and how to reach them. Some of the most common configuration mistakes that happen when initially learning OSPF revolve around these neighbor relationships; if they don’t exist then the devices will be limited to only the locally (and limited to each individual device) reachable networks.

Because the OSPF neighbor relationship is so important to the operation of OSPF it is best to start with OSPF neighborship (this is a common term used to refer to the relationship between OSPF devices).

When OSPF is configured on a router the first step that must be taken is to enter into OSPF router configuration mode. This is done with the router ospf process-id command. It is important to note that the process-id that is used in this command is significant to this router only and does not need to match any parameter on any other device; it is only used to identify this OSPF instance on the current device.

R1#configure terminal
R1(config)#router ospf 10
R1(config-router)#

The next step is to configure which networks will be included into the OSPF process and into which OSPF area they will be included; this is done with the network command.

R1(config-router)#network 192.168.1.0 0.0.0.255 area 0

One of the most common mistakes is to use an incorrect wildcard mask. Generally the wildcard mask that is used is the inverse mask of the interface subnet mask. For example, if an interface is configured with an IP address of 192.168.1.1 with a subnet mask of 255.255.255.0, the inverse wildcard mask would be 0.0.0.255 which includes the addresses from 192.168.1.0 through 192.168.1.255 (this is not intended as a subnet tutorial so if this if foreign then some additional work on subnetting may be required). The second part of this command is specifying which OSPF area these networks will be included in.

In this case the networks from 192.168.1.0 to 192.168.1.255 will be included in OSPF area 0. Now assuming that no other changes have been made to OSPF, the local device is ready to bring up a neighborship with a connected router which is also configured with OSPF.

There are a couple of rules that must be followed for a basic OSPF (for IPv4) neighborship to form:

  • Both OSPF devices must be on the same subnet
  • Both OSFP devices must be in the same Area
  • Both OSPF devices must share the same hello and dead intervals
  • Both OSPF devices must have unique Router-IDs (RID)

If any of these rules are not met then the OSPF neighborship will not form.

The last thing to check for is if all of these other things have been verified. To do this you check the OSPF network type used on the connected interface. Generally when a new networking student is learning OSPF they are connecting the devices using Ethernet; Ethernet (by default) supports multicast/broadcast which is used by OSPF. Some other technologies either don’t support multicast/broadcast or don’t by default. In these cases it may be required to either change the OSPF network-type OR to manually configure OSPF neighbors. Both of these are outside the scope of this article but should point you in the right direction if a connection other than Ethernet is being used.

Summary

When getting into networking it is important to get at least a basic handle on how the different protocols operate and OSPF is no different. Many different companies utilize OSPF and it can be a major factor in choosing the right entry level networking individual when a position is open. Hopefully the content in this article will help the reader understand OSPF a little better or solve the problem they are currently having.

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