Hello all :)
So, the way summarisation bounds the EIGRP query domain is as follows.
Just image this simple topology
Ie, if you run a 10.0.0.0 network in siteA and 18.104.22.168 in siteB with auto-summary disabled and no manual summarisation (for simplicity, all networks subnetted to /16s), then each router in the network in site A and Site B would have a topology table entry for each individual /16 in net10 and net11. Query would go everywhere.
If network 10.1.0.0 in siteA went down, and auto or manual summarisation was enabled, the query would only get to the summary border router (lets say siteBrtr1 that has 10 and 11 networks on) and query its first neighbor in SiteB (siteBrtr2) and as this router would not have a topology entry for the /16 (10.1.0.0) and only the /8 (10.0.0.0) for net10 thus that router would not query its neigbors in site2.
So, Can I make the following statement on how this works.
"A query only gets sent by a router to its neighbors (apart from the successor) if that router has an exact entry for the prefix/length in its topology table? "
Is that statement correct? This is important as ACLs and other stuff, could affect the query range, if you have routers in the path to the destination that blocks this prefix, that could also limit the query range for a network.
Many thx indeed,
your statement is correct. The underlying reason is that a query in fact also is informing about a network being unreachable.
If an EIGRP router gets an unreachable statement about a network it never had, "it is not bothered". In this case nothing is removed from the routing table, so no need to ask neighbors about reachability.
If the network existed, however, then there is a real change in the routing table and the EIGRP router needs to evaluate, if a substitute exists. In the case of a feasible successor, the answer is yes and can be given locally from the topology database. If no fesible successor exists the neighbors need to be queried.
Hope this helps!