What is Null Interface in a Cisco Router | Configuration Example

The Null interface Router is a “bit bucket” p or can be considered as a “black hole “interface where all unwanted traffic is sent to be discarded. It is mostly used for filtering unwanted traffic because traffic can be discarded simply by routing it to the null interface.

Although the same objective can be achieved through using the access-lists they require more CPU usage. Unlike loopback interfaces, there could be only one null interface and it accepts only one configuration command “no IP unreachable”.

The Null interface is a virtual interface and is always up these interfaces never forward or receive traffic rather packets routed to this interface are dropped. This is a Cisco router mechanism that helps to present routing loops, EIGRP (Enhanced interior gateway routing protocol creates a route to Null0 when it summarizes a group of routes.

There is no need to assign an IP address to the Null interface rather creates a static route and put the next-hop Null interface (Null0).

Static Route to Null Interface
Filtering with Null Interface

 

If we do not want the user on (10.10.2.0) to reach (10.10. 3.0) we can add a static route on router 2 that will send all the traffic destined for (10.10. 3.0) network to the null interface.

The null interface is usually used for preventing the routing loops, when we configure summarization an OSPF ABR (Area Border Router) and ASBR (Autonomous Border router) generates the summary routes but they are not real in redirects them to Null interface and it’s called discard routes. A Null route is a route where matching packets are dropped instead of forwarding them to any destination.

Configuration example

Configuring Null interface

 

In order to verify the configuration of the null interface use “show interface null0” command as shown below.

Configuration of Null Interface

 

During the DOS attack the null route can be temporarily placed as next-hop which will cause that device to drop all the traffic for all unwanted traffic.

Same Like Cisco System Unix system also uses null interfaces where all the output send to /dev/null disappears, In Linux world, it is not always interested to see the output of the command.

Null Interface Router on Linux
Null interface on Linux

 

The Summary address command is used only in ASBRs (Autonomous border routers) since the connected routes are being redistributed the router itself uses null 0 as a discard route.

Also, read…


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