In large enterprise networks, many distribution devices have more than 200 interfaces. Before the introduction of the Default Passive Interfaces feature, routing information could be obtained from these interfaces in these ways:
Configure a routing protocol such as Open Shortest Path First (OSPF) on the backbone interfaces and redistribute connected interfaces.
Configure a routing protocol on all interfaces and manually set most of them as passive.
Network operators might not always be able to summarize type 5 link-state advertisements (LSAs) at the device level where redistribution occurs, as in the first possibility. Thus, a large number of type 5 LSAs can be flooded over the domain.
In the second possibility, large type 1 LSAs might be flooded over the domain. The Area Border Router (ABR) creates type 3 LSAs, one for each type 1 LSA, and floods them to the backbone. You can, however, have unique summarization at the ABR level, which injects only one summary route into the backbone, thereby reducing the processing overhead.
Before the introduction of the Default Passive Interfaces feature, you could configure the routing protocol on all interfaces and manually set the passive-interface router configuration command on interfaces where adjacencies were not desired. But in some networks, this solution meant configuring 200 or more passive interfaces. The Default Passive Interfaces feature solved this problem by allowing all interfaces to be set as passive by default. You can set all interfaces as passive by default by using the passive-interface default command and then configure individual interfaces where adjacencies are desired using the no passive-interface command.
The Default Passive Interfaces feature simplifies the configuration of distribution devices and allows the network administrator to obtain routing information from interfaces in ISPs and large enterprise networks.