Why do I have an interface with ifDescr = Null0 in the ifTable?
A. As of Cisco IOS® Software release 12.0, there is an interface with ifDescr Null0 showing up in the ifTable. The null interface, Null0, is a virtual network interface (similar to the loopback interface). While traffic to the loopback interface is directed to the router itself, traffic sent to the null interface is discarded. The null interface might not be configured with an address. Traffic can only be sent to this interface by configuring a static route where the next hop is the Null0 interface. This is done to create a route to an aggregate network that can then be announced through the Border Gateway Protocol (BGP), or to ensure that traffic to a particular range of addresses is not propagated through the router, perhaps for security purposes. The router always has a single null interface, Null0. By default, a packet sent to the null interface causes the router to respond by sending an Internet Control Message Protocol (ICMP) unreachable message to the packet’s source IP address.
