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In which case is

ip route 10.1.0.0 255.255.255.0 null0 used?

Thanks in advance.


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3 Answers 3

It drops all the traffic that doesn't have a more specific route. It is used to prevent routing loops and to put routes in the routing table permanently so they can be announced with BGP.

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is that null0 networks are like loopback network? –  Trojan Apr 19 at 12:20
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Not quite. I can make a loopback interface on a router (Loopback0, for instance) and make it reachable from a remote network. Loopbacks are just virtual network interfaces. Null0 is more like the vast emptiness of space; data sent to Null0 is discarded and never thought of again. –  Avery Abbott Apr 19 at 13:57

Null 0 is a virtual interface that’s used to guarantee routes make it into the RIB; remember, routes must have a next-hop in order to make it into the RIB. When a packet arrives at a device that has a route to Null 0 and nowhere else, it will discard it similar to how an ACL would. Think of this as the virtual interface where packets go to die efficiently.

Most routing protocols, such as EIGRP, automatically generate a Null 0 route when summarizing addresses.

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Null 0 can also be a good alternative to an Access Control List(s) with the benefit of lower CPU overhead (depending on the complexity of requirements). –  one.time Apr 21 at 1:02

The null 0 interface can be seen as the same bitbucket as a /dev/null device in UNIX/Linux. Everything that ends up there will just be dropped.

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