If auto-summarization is enabled, the router will automatically summarize routes at classful network boundary and advertise a single route for an entire Class A, B, or C network. However, if your lab only consists of classful networks, but not subnets, you will not see any difference by turning the feature on or off.
KPLAB
Phil, Auto-summary can be dangerous. The end result is that RIP cannot be used with subnets of a classful address when those subnets are reached through different interfaces. The same goes for IGRP and EIGRP w/ autosummary on, because as kplab said, autosummary summarizes routes at the classful network boundary. First, imagine three routers in a line, connected via serial point-to-point links. r1------------------------------r2-----------------------------------------r3 s0/0 s0/1 s0/0
r1 has an ethernet network address of 10.1.1.0 /24 r3 has an ethernet network address of 10.2.2.0 /24
RIP always auto-summarizes. It sees a 10.1.1.0 network as 10.0.0.0. It autosummarizes based on the network number. Just like when you configure rip, you would summarize the number 10.1.1.0 to 10.0.0.0. You enter: router rip network 10.0.0.0
Even if you entered: router rip network 10.1.1.0
It would appear in the running-config as router rip
10.0.0.0rip and igrp autosummarize and it CANNOT be turned off. Only with EIGRP can it be turned off.
The problem occurs when r1 and r3 both send routing updates advertising their ethernet network numbers. r2 recieves the update, and the routing table looks like R 10.0.0.0 via s0/1 via s0/0
Now we can see that if a host on r2 ethernet tries to communicate with, say, a host on r1 ethernet, a problem occurs. r2 actually load balances out s0/0 and s0/1.
The only way to solve this problem is to migrate to eigrp and use no auto-summary
pretty cool, eh, alan
can anyone explain to me what the consequences of turning of auto-summarisation on eigrp are? I've read the cisco books on the subject but I just can't get my head around it. I've also setup a lab and turned it on/off and don't see any changes in what routers are advertised. help!
Yeah, it turns EIGRP into a sucky protocol.
The default, 'auto-summary' summarizes on classful boundaries. This blows. Turn it off.
Jonathan
I think you would want to turn this off if you are using vlsm with discontinous networks.. This is because a subnet may not "fit" into the summarization
Maybe someone else can correct me if I am wrong.
Well, I am writing a CCIE R/S lab study guide, and I am tired of hearing long, drawn out reasons why you should do something, why not write to the audience: one way good, other way nightmarishly suckworthy.
I am at 165 pages, and it is growing, I should have something worth distributing in about a month or two.
The problem I am running in to is that my life is about to become all about Cisco CallManager and Unity... So, routing and switching might have to take a backseat for awhile...
Jonathan
the kind of no nonsense straight tawking i like
Hi Jonathan, sounds very ambitious.
Are you wanting people to proof read (proof study) your guide? I'd be interested if you are, let me know and I'll send you my email address.
Not particularly.
I discovered awhile ago that the only way I can thouroughly learn something is to write down everything I know in an organized way, the result tends to be a book.
Not yet.
The goal of this thing is still for personal use, it isn't at a level where it would be particularly useful to anyone else.
And my writing style is a tad, how shall we say this, blunt.
If there is a common mistake I have found myself making, I have a tendency to scream at myself in the document (for example, including the word 'broadcast' on the end of frame-relay map statements), so I will remember to do something. This tends to come across as condescending and rude.
When it is ready to be proof-read, I will post a request for takers on here...
Jonathan
Hope we get to see it one day man I wish I had the time to get that deep into things. Heh 7 kids and one heck of a good woman take most of mine but then I shouldnt brag. ;) But the best way to learn a subject is to become a teacher of that subject so I see your method here. Good luck
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