Equipment Support > Hardware By Brand > Cisco > [HELP] Why use the no auto-summary command for EIGRP

[HELP] Why use the no auto-summary command for EIGRP

i find it hard to believe that you've read your books and that situation hasn't been explained.

»supportforums.cisco.com/message/620091

of course -- in a nutshell -- imagine a campus network. the entire network functions within the 10.0.0.0/8 space internally. you're going to use vlsm to break the network up into different subnets as needed (/30's or /31's for point-to-point links, /23 or /24 for user access, etc.).
routing protocols automatically summarize the networks they advertise (i.e. your router wouldn't care if you had 10.0.1.0/24, 10.1.0.0/24, and 10.100.1.0/24 in your routing table, it would advertise one route -- 10.0.0.0/8). when every router is trying to do this -- they all eventually begin to get confused because they all have routes to the same destination (10.0.0.0/8) but no "more specific" prefix information is carried.

q.
--
"...if I in my north room dance naked, grotesquely before my mirror waving my shirt round my head and singing softly to myself..."

reply to JDmailNY
You would use no auto-summary when you require carrying traffic that originates from a subnetted subnet to keep its classless address across the neighbor router. It's usually not an issue when neighbor routers share the same subnetted address space on their interfaces.

I agree with tubbynet.. I would suggest thoroughly reviewing that lesson to understand the concept behind VLSM and auto-summarization, especially when dealing with RIP, OSPF and EIGRP.

thank you