Cisco IOS Syslog Messages

Hello Allal

The Null0 route is used for various reasons in the routing table. Any routing entry that has an exit interface of Null0 will drop packets to that destination. Using Null0 you are explicitly stating that any packets destined to this specific destination will be dropped.

Null0 interfaces in the routing table are used for various reasons. One of the most common is when EIGRP summerization is enabled. EIGRP will advertise a summary route to other routers, but at the same time, will have a routing table entry to the summary route pointing to the Null0 interface. An example of such an entry in the routing table can be seen below:

R1#show ip route eigrp is variably subnetted, 3 subnets, 2 masks
D is a summary, 00:01:38, Null0

This is done to avoid routing loops. In the absence of any more specific route, the above routing table entry would cause any packets destined for this subnet to be explicitly dropped. More about this type of summarization (and why the Null0 interface is used) can be found in the following lesson:

I hope this has been helpful!