summary routes to null zero

Unanswered Question
Oct 25th, 2008

hi all, i have seen that when you put a summary route on a router, it creates a route to null zero, why is this ?

I have this problem too.
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vishwancc Sat, 10/25/2008 - 18:34

Hi ,

As per my understanding there are 2 reason.

1.If you are using a summary route and the route inside the network flapped ,it will not trigger any

reculation to the outside networks since you are advertising the summary route and not the specific route which flapped

which is very handy when we look at BGP with 10000 of route and if 1 route flap due to link it will not trigger

update and recalculation for the whole network.

2.If you receive any packet for subnetwork that falls in the network advertise by the summary ,but that subnetwork is not present in the routing table.

the router will send it to null0.



Istvan_Rabai Sun, 10/26/2008 - 11:25


A little addition to the previous post:

A summary route to null0 can prevent a routing loop to form.

Imagine a situation where there is a default route as well in the routing table.

If a packet comes in destined for a subnet that resides in the summary range, and there is no specific prefix for it in the routing table and you don't have a summary route pointing to null0, what happens?

You guessed right:

It will go out on the default route.

And what if the default route directly or indirectly points back to the router where the packet came from? That router will route the packet back to this router again.

Well, this is a nice routing loop.

So, if you have a summary route pointing to the null0 interface, this packet will have a match on it and will be dropped (if there is no explicit route for the more specific subnet in the routing table).



Marwan ALshawi Sun, 10/26/2008 - 19:54

hi carl

the two above posts great

to simplify it for you

the router who generate the summary route wil point to null0

which prevent loop, let the router it self to know that the summary done on this router

finally this null0 can be used to block routing for spicific network with a term called black holing

for example if u have summary to a network and there is part of the summarized network not existed

the router will recive route for that not existed network but then will drop it as it is not connected

good luck

hope this helps


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