eigrp "no auto-summary"

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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!




Re: eigrp "no auto-summary"


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
http://www.kp-lab.com - Free CCNA Study Guide



Re: eigrp "no auto-summary"


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
10.0.0.0

rip 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



Re: eigrp "no auto-summary"


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.




On 5/18/05 8:59 AM, in article USHie.7647$WQ3.1318@newsfe5-gui.ntli.net,

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Re: eigrp "no auto-summary"



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Yeah, it turns EIGRP into a sucky protocol.

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The default, 'auto-summary' summarizes on classful boundaries. This blows.
Turn it off.




Jonathan




Re: eigrp "no auto-summary"


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the kind of no nonsense straight tawking i like




Re: eigrp "no auto-summary"



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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




Re: eigrp "no auto-summary"



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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.




Re: eigrp "no auto-summary"



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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.

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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




Re: eigrp "no auto-summary"


On Fri, 20 May 2005 22:37:07 -0500, "Jonathan"

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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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