OSPF IV: There is No “I” in Team: More about DR’s

DR vs. No DR

One of the unique features of OSPF concerns neighbor relationships across multiaccess networks, such as Ethernet LANs and certain types of WAN’s such as Frame-Relay and ATM (no, not the cash machines at banks).  Remember, neighbor relationships form between connected neighbors across links, so consider the diagrams above to get an idea of what this looks like. Here we have listed six routers, and the math required to calculate the number of relationships is N(N-1)/2, where N is the number of devices.  Solving for 6 in this case creates the values of 6 * 5, yielding 30, and dividing that by 2 results in 15, which is a lot between so few devices, and staggering between many.  To simplify this, OSPF has a single peer on multiaccess networks called the Designated Router, or DR.  Note in the diagram how the 15 relationships is reduced to 5 using a DR, and in reality there are 5 more with a secondary DR, called the BDR.  In keeping with the highly regimented structure of the protocol, all messages, updates, and so forth take place between peers and the DR, and NOT with one another.  The purpose of the BDR is to take over if anything happens to the DR.

As is the case with many other network protocols, there is an election process to determine the DR and BDR roles on the multiaccess networks, and seldom is this optimal.  I have personally had to deal with suboptimal DR selection in networks and labs I have worked on, especially when dealing with redistribution (the process of sharing routes between various routing sources).  Each router on the OSPF multiaccess network has a default priority of 1, which usually results in a tie in the election process, and the highest numerical Router-ID wins if that is the case.  To set the priority manually, use the ip ospf priority <0-255>  command on the interface, understanding that the higher the priority, the better (I routinely use 200 for the DR and 190 for the BDR).  To remove a router from the election process, just specify zero (0) as the priority, which is useful in hub-and-spoke topologies such as Frame-Relay (having a spoke router as the DR or BDR is not helpful at all).

Let me share a quick word on WAN topologies in OSPF because they can drive you crazy at times.  Frame-Relay and ATM do not forward broadcasts naturally, and special configuration is required is you want that functionality.  If you have to work with non-broadcast links, use the ospf neighbor command under the OSPF process.

Next time, we will wrap up our OSPF discussion by looking at route selection.

– Joe


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