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Routing Null0 configuration

Posted on 2008-10-18
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Last Modified: 2012-08-13
Hello all,
i have some routes on my router connected to Internet and i have multiple route to the inside of my network
for each subnet routed, i have the ip route line and 2 more routes to Null0. It's public IP and in the example i used private IP.
Can someone please explain me for what we have this 2 routes to Null0 and do i really need them ?
example:
ip route 172.16.2.0 255.255.255.0 192.168.1.1
ip route 172.16.2.0 255.255.255.255 Null0
ip route 172.16.2.255 255.255.255.255 Null0

thanks
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Question by:lemaitre75
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by:kdearing
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The Null interface is typically used for preventing routing loops.

http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml
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by:Don Johnston
Don Johnston earned 150 total points
ID: 22749858
The function of the Null next hop is to allow creating a static route where the traffic won't be forwarded.

This could be, as previously stated, to prevent a routing loop. It can also be used when a route is needed in the routing table, but it won't be used.

In your case it could be that it was needed in the past for some purpose but isn't required anymore. Without seeing the rest of the config and knowing the topology of the network, it's impossible to say for certain.
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rexxus earned 300 total points
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The routes to Null0 are either to enable the routes to appear in the internal routing protocol route table so that they can be propagated to an external routing protocol such as BGP.  They would no longer be needed with the ip route 172.16.2.0 255.255.255.0 192.168.1.1 configuration in place.

The interesting thing is that the routes are for the network and broadcast address of the network, these were possibly put in place to stop some sort of mapping scan or smurf attack.  There are other commands, so as no "no ip directed-broadcast" on the interface that can perform this task in place of the static route.
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