Routing - The Null0 Interface

The Null0 interface is a special virtual interface on a Cisco IOS device that can be used to achieve various goals especially when it comes to routing. It is possible to create a static route within the Routing Table that points to the Null0 interface. Any packets that match this route will be dropped. This process is also known as "blackholing."

Note that this process differs from traffic simply not being matched in the routing table and being dropped; in this case, the traffic is explicitly matched and then purposefully dropped.

There are several reasons to use such a route pointing to the Null0 interface. These include:

• To force the advertisement of a particular route using BGP.
• To summarize routes and prevent routing loops - When summarizing routes, the null0 interface can be used as the next hop for the summary route. This ensures that any traffic matching the summarized route but not any of the specific routes within that summary is discarded. This is useful for keeping the routing table concise and ensuring only valid routes are propagated.
• To create controlled packet drops - It allows administrators to intentionally discard specific traffic that matches certain criteria. For example, if certain packets need to be dropped for security or policy reasons.