This topic describes the null interface, which is always up, and provides an alternative to access lists for filtering unwanted traffic on the Cisco 8000 Series Router.
The null interface is a virtual interface that
-
remains permanently active
-
discards all received traffic, and
-
provides an alternative method for filtering undesired network traffic.
A null interface functions similarly to the null devices available on most operating systems. 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 that is involved with using access lists by directing undesired network traffic to the null interface.
Key characteristics of null interfaces
The system supports several key characteristics and configuration options for null interfaces:
-
The only interface configuration command that you can specify for the null interface is the ipv4 unreachables command.
-
With the ipv4 unreachables command, if the software receives a nonbroadcast packet destined for itself that uses a protocol it does not recognize, it sends an Internet Control Message Protocol (ICMP) protocol unreachable message to the source.
-
If the software receives a datagram that it cannot deliver to its ultimate destination because it knows of no route to the destination address, it replies to the originator of that datagram with an ICMP host unreachable message.
-
By default, the system enables the ipv4 unreachables command. If you do not want ICMP to send protocol unreachable messages, configure the ipv4 icmp unreachable disable command.
-
By default, the system creates the Null 0 interface during the boot process and you cannot remove it. You can configure the ipv4 unreachables command for this interface, but most configuration is unnecessary because this interface discards all packets that the system sends to it.
-
Use the show interfaces null0 command to display the Null 0 interface.