Thanks for your comment.I have gone thru the document you provided. Thanks again.
But you mentioned "ip summary-address eigrp 1 0.0.0.0 0.0.0.0". Won't this advertise default network towards Router B? which cisco says it will create Null Route. (mentioned in the link you provided).
My contention is if network in Router A (172.16.1.0) goes down, will it create query message towards Router B and will propogate towards Router D and Router E?
I am not able to understand by which means Router reply the query? For example i assume if a query comes to Router D, it will check in its topology table for details for 172.16.1.0/24, if neither Successor / Feasible succesor is avaialble it goes to Active state and query its neighbour. It want to stops this query goes from Router B using summarisation?
my understanding is that the router receiving a summary route in this case a default route summary route when receives the Query can answer back with a Reply telling it is not a possible backup path for the route.
This should be the reason why route summarization helps to limit EIGRP query scope.
Something similar happens with the EIGRP stub router feature.
So you need to summarize out the interface of the core router where you want to stop EIGRP queries
Using a default route summary route has the effect to stop every possible Query.
1. When a query reaches a router i know that i will see whether this router is having route for the queried prefix in either as successor or Feasibile Successor. If both are not avaialble it will start querying its neighbour and move to Active state.
But When it is having summarized Route for the queried prefix, whether it will do this same activit or it will simply reply the router telling it is not having the route and donot go to active state?
when a router receives a Query for a route from a neighbor performs a basic check:
it sees if it has a detailed route matching the requested route in the EIGRP topology table.
If it hasn't and the route is contained in a less specific route and that route is learned only from the Query originator the router is allowed to answer with a negative Reply stopping Query propagation.
The idea is that if no detailed information has been learned downstream up to now, and the only possible match is a summary route that has come from the same interface that received the Query there is no chance to get an alternate path by propagating the Query.
So this router RB in your scenario can simply answers back "no alternate path here".
if on interface between RA and RB RB receives only a default router whatever Query RA sends out RB will answer with a negative Reply.
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