NBMA Environments

NBMA Environments
Earlier, we mentioned that there are three types of networks: broadcast multi-access, non-broadcast
multi-access, and point-to-point. Although NBMA requires somewhat more configuration to make
OSPF operational, it also gives you the option of deciding how you want it to behave.
With extended configurations on NBMA interfaces, an administrator can cause OSPF to
behave as if it were running on one of the following five network types:
 Broadcast
 Non-broadcast
156 Chapter 5  OSPF Operation in a Single Area
 Point-to-point
 Point-to-multipoint
 Point-to-multipoint non-broadcast
It is important for you to know what the Hello and Dead intervals are for each of the five
network types as well as whether the network type will elect a designated router (DR) and a
backup designated router (BDR). If you change the network type, you must make sure all other
interfaces on that particular network segment have at least the same Hello and Dead interval,
or they will not communicate. It’s a good idea if you change the network type on one interface
on the network segment to change all the other network types on all the other interfaces to
match. This is suggested but not required. You could just change the Hello and Dead intervals
for the interfaces to the same as the interface with the different network type. The only issue
with this solution is dependent upon the network type—a DR/BDR may be elected or not.
Table 5.3 summarizes the important differences between the five configurable OSPF network
types.