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Null Interface

What is a Null Interface ?

By Atul Sharma

What is a Null Interface ?

Null interface is not a physical interface. Its a virtual interface where you can route traffic, but want packets destined for a particular network to be dropped. Directing packets to the null0 interface will prevent the router from sending an Internet Control Message Protocol (ICMP) network unreachable message when it receives a packet with an unknown destination. A situation in which you might want to use the null0 interface is when you have configured a default network, but want packets destined for a particular network to be dropped.

If a static route is configured for the network to the null0 interface, the packets will be dropped without the ICMP message or without having to configure an access list. This interface is always up and can never forward or receive traffic; encapsulation always fails. The null interface provides an alternative method of filtering traffic. You can avoid the overhead involved with using access lists by directing undesired network traffic to the null interface.


Introduction to Null Interface

Null interfaces are completely software-like logical interfaces. Null interfaces are always up. However, they can neither forward data packets nor have IP addresses and link layer protocols configured on them. With a null interface specified as the next hop of a static route to a specific network segment, any packets routed to the network segment are dropped. Null interface provides you a way to filter packets. That is, you can simply transmit unwanted traffic to a null interface rather than applying ACLs.

For example, by executing the?ip route-static null 0?command (which configures a static route leading to a null interface), you can have all the packets destined to the network segment discarded.

Configuring a Null Interface

Follow these steps to configure a null interface:

To do?

Use the command?


Enter system view



Enter null interface view

interface null 0


Null0 interface is the default null interface on a switch. It can neither be created nor removed.

Configure a description for the null interface



By default, the description of a null interface is its interface name followed by the word?interface, for example,?Null0 Interface.

Display information about the null interface

display interface null?[??]