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A null interface is a virtual interface that discards IP packets and is used to prevent routing loops from occurring in the network. You do not assign the null interface an IP address.
The Null0 interface does not forward packets. All packets sent to this interface are discarded. The Null0 interface is applied in two situations:
Loop prevention, the Null0 interface is typically used to prevent routing loops. For example, during route aggregation, a route to the Null0 interface is always created.
Traffic filtering, the Null0 interface provides an optional method for filtering traffic. Unnecessary packets are sent to the Null0 interface to avoid using an Access Control List (ACL). Both the Null0 interface and ACL can be used to filter traffic as follows. Using a Null0 interface to filter traffic is more efficient and faster.
Using the Null0 interface for packet filtering only requires a route, but using the ACL for packet filtering requires an ACL rule to be configured and then applied to the corresponding interface on a router. The Null0 interface can filter only the router-based traffic, whereas the ACL can filter both the router-based and interface-based traffic.
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