3.1 Description

The null interface is a virtual interface for discarding packets that does not depend on physical lines. You can discard packets by specifying the null interface as the output destination of specific traffic.

The null interface is always enabled and neither forwards nor receives traffic. The null interface does not report the discarding of packets to the source by sending an ICMP Unreachable message. The null interface is not used to discard multicast packets.

With the null interface, you can restrict traffic to a specific network or terminal that passes through the Switch. In the following figure, all the traffic sent to network B via the Switch is directed to the null interface so that it will be discarded.

Figure 3-1: Null Interface Network Configuration

The null interface function is part of static routing. When you use the null interface to discard packets, you need to specify a static route to set the null interface as the output destination.

When the Switch searches for a route, the Switch internally discards and does not forward packets it determines are directed to the null interface (packets that are routed to the null interface based on a static route).

For more information about static routing and routing, see 9 Static routing (IPv4)"~"13 BGP4).

The switch also provides filtering to discard packets based on conditions specified for each interface. However, the null interface is useful because it allows the Switch to discard unnecessary packets all at once when you specify simply a static route to the null interface for specific traffic.

The following table describes the function that discards packets when both the null interface and filtering are used.

Table 3-1 Discarded part of the packet when Null interfaces and filtering functions are used

Routing information	Filtering settings	Operation	Discard mechanism
Directed to the null interface	Forward	Discard	Null interface
	Discard	Discard	Filtering
Directed to a non-null interface Directed to a non-null interface	Forward	Forward	-
	Discard	Discard	Filtering