I am currently studying the BSCI material and have a question about limiting EIGRP query range using route summarization.
Here is the example given:
Router B advertises the route summary of 172.16.0.0/16 to Router A. If the route for router B to 172.16.30.0/24 goes down, this is advertised to Router A as being down, but since Router A is not aware of this route, no changes are made to its routing table. This is because it is aware of the route to 172.16.0.0/16 through Router B using route summarization, rather than having the individual routes.
My question is, what is the advantage other than processing and bandwidth to this? Queries from Router A for 172.16.30.0/24 will still be passed to Router B, which will then return a Destination Path Unreachable message to the host on Router A. Is the main reason for this to avoid Router A from being put in an active state while it tries to find a new path to 172.16.30.0/16? If that is the case, then I understand the reasoning behind using summarization. Or am I way off on my understanding? Thanks.
I want to be very clear for you:
When Router B advertises 172.16.0.0/16 to Router A, it will not advertise any changes of route 172.16.30.0/24 to Router A.
One of the advantages of summarization is that frequent updates caused by flapping routes are not propagated towards routers on the other side of the summarizing router.
Router B will send an advertisement to router A to remove 172.16.0.0/16 from Router A's routing table when all the more specific routes embraced by summarization are down.
When 172.16.30.0/24 goes down, Router B looks for Feasible Successors in its topology table. If there is no Feasible Successors, Router B will send a query(! not update) to Router A about 172.16.30/24.
A router forwards a query to other routers only if it has an exact match in the routing table.
Router A has only the 172.16.0.0/16 prefix in the routing table (from Router B).
As Router A does not have the 172.16.30.0/24 prefix in its routing table, it will answer the query to Router B that it has no route to that destination. Propagation of the query stops here (see Key point).
The query will stop at Router A and will not be propagated further, behind Router A.
This is how limiting query range works using route summarization.