when you use command ip route 10.1.0.0 255.255.0.0 null0
then it is the static route which will apear in your routing table...and when the packet whith destination 10.1.0.0 will receive on router then it will send that packet to the null0 interface it means it is going to discard the packet...so this is used to avoid some time loops by discarding the packet ...here you are not sending summary to the null0 interface ...but you are making routing disision using the static route to null0 interface...
The idea behind this is simple and let me try to put it here again...
You summarise the routes and advertise it WHY so that the routing table of the peer routers will have less entries in routin table and need not be populated with all routes which are in that summarised route.
Now WHAT happens if any of the nodes behind the peer routers try to reach any of the specific route in that summarised route it will be fwded to your distribution router on which you have summarised teh routes and from there it will look for most specific route from that summarised route NOW lets assume that specific route is down then WHAT will happen it will not find any entry in routing table and either will be dropped or incase you have a default route it will be fwded to that default router and from there again it will check the summarised route and will be fwded to the distribution router and this will go on and on and on.....so to stop this if you have a static route for summarised route pointing to null0 interface and if most specific route within that summarised route is down so any request for that destination will be fwded to null0 static route because that is the next best match and will be dropped.
So that is the intention behind this configuration to avoid possible routing loop.
it means when you send summery to access layer and when access layer will send any packet for the destination include in summary to the distribution layer...if the destination for the packet is down, at that time if there is default route ...then packet will send to that interface even the destination is down which is not a good right that is why you are using the null0 am i right???
You got the point!! So if there is a default route packet will be fwded to the interface where default route is pointing as most specific match is not available because it is down. This is indeed not preferrable so that is the reason null0 is used to avoid that behavior.
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