Final answer:
The passive-interface default command in OSPF configures all interfaces on a router as passive interfaces by default. Passive interfaces do not send or receive OSPF routing updates, reducing OSPF traffic. It is useful for preventing unnecessary OSPF traffic on certain interfaces.
Step-by-step explanation:
The passive-interface default command is used in OSPF (Open Shortest Path First) routing protocol to configure an interface as a passive interface by default. When this command is issued on a router, all interfaces on that router are set as passive interfaces by default, unless explicitly configured otherwise.
A passive interface does not send or receive OSPF routing updates, which means that it does not participate in the OSPF routing process. This can be useful when you want to prevent unnecessary OSPF traffic on certain interfaces, such as WAN links or interfaces connected to non-OSPF routers.
By issuing the passive-interface default command on a router that is configured for OSPF, you are essentially making all interfaces on that router passive interfaces by default. This can be helpful in larger networks where you want to limit the number of active OSPF interfaces, improve network performance, and reduce unnecessary OSPF traffic.