10-25-2008 10:49 AM - edited 03-06-2019 02:09 AM
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 ?
10-25-2008 06:34 PM
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.
Chao
Vishwa
10-26-2008 11:25 AM
Hi,
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).
Cheers.
Istvan
10-26-2008 07:54 PM
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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