Usage and working of Null0

Unanswered Question
Feb 10th, 2009

Hi Guys,

I m confused about the usage and working of the routes pointed to null 0.

I understand that it is used for route black holing but i m confused the usage of it in the eigrp or ospf when summarization is used,the IOS creates automatically a route pointing to null 0 for the summarized route i want to know why this is created,i have known it is created for loop prevention i want to know how it does it.



I have this problem too.
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adamclarkuk_2 Tue, 02/10/2009 - 01:53

Hi Mahmood

So when route summaruzation is configured in OSPF, as you stated a NULLO entry is added. Any packets destined for the summarized routes will have a longer match in the FIB and are routed to them, anything else is dumped via NULL0.

If you where summarizing 3 /26 via a /24, then you dont want to be passing the packets for the 1 /26 you dont know about otherwise they could end up coming back to you and then you pass them on again, hense the loop aviodance.

mahmoodmkl Tue, 02/10/2009 - 22:01


I didnt get u in u r second para.

Can u explain it with a example.



Istvan_Rabai Tue, 02/10/2009 - 22:59

Hi Mahmood,

Imagine that there is a default route ( ) in the routing table and suppose OSPF did not put the summary route to Null0 into the routing table.

What happens if a packet comes in to this router that matches a component subnet of the summary route, but for some reason that specific component subnet is not reachable on this router? (Let's say for link being down).

The packet will match the default route.

The default route may direct the packet out an interface and to a router that may eventually send the packet back to this router. This is a routing loop that in some specific circumstances can happen.

To the contrary, when a summary route is created on this router pointing to Null0 the following happens:

If the incoming packet does not match a specific component prefix within the summary, then it will surely match the summary route and eventually will be dropped, and it will not form a routing loop.

I hope I was explaning understandably:)



mahmoodmkl Wed, 02/11/2009 - 00:35


Thanks for the reply.

Can u please explain with some example by showing the summarization and the process.



adamclarkuk_2 Wed, 02/11/2009 - 04:02

Hi Mahmood

So I have the following networks /24 /24 /24 /24 /24 /24 /24

So I am missing /24 but still want to summarize and am using eigrp with auto summary turned off.

I summarize these routes using /21 which equals



So all my networks are covered and I apply the interface level command :-

ip summary-address eigrp 10

now as you said I have the NULL route in the RIB. Now what happens when a router needs to get to /24, they are going to send it to me right, but I dont know about it and dump it. If I didn't have the NULL route, eigrp will start to send query packets to the other routers and generating loads of un wanted traffic and if a default route was in the routing table, it is now going to send it out there. If your a single armed router, ( which is anther good place to summarize ), then your default route would be the router that just sent the packet to you, and you will send it back to him and so on and so on.

mahmoodmkl Sat, 02/14/2009 - 02:15


Thanks for u r reply.

How does the encompass all the subnets i m missing a little thing here...please explain.

Here we i think super-netting i m confused about it i.e how it works.




the summary address does encompass all the subnets, it will advertise all addresses from to However is not one of the ranges that are in the summary.

Routing table output

C is directly connected, Loopback5 is subnetted, 1 subnets

C is directly connected, Serial2/0.23

C is directly connected, Loopback6

C is directly connected, Loopback7

C is directly connected, Loopback1

C is directly connected, Loopback2

C is directly connected, Loopback3

D is a summary, 00:14:49, Null0


so the routing table has a summary route to null0 with a /21 mask and has /24 routes for the ranges apart from meaning any pakets arriving for will be dropped as there is no longer match but there is for the other routes. Remember the longest match is the first deciding factor on route decisions.

Hope this helps


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