hi minumathur...
you will clear idea regarding to the NULL0 form this explanation...
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
rate this post if it helps
regards
Devang
