Re: !-RE: Auto Summary Revisited

Thank you Karim/Bob/Rendezvous-

I just labbed up the scenario mentioned by Bob and you are absolutely right
about the auto-summary. Here is my understanding, please correct me if I am
wrong!!

1. EIGRP- When auto-summary is enabled on EIGRP, it will summarise route to
classful boundary when advertising to another interface. So what it means is
that, it will start summarising the network when it crosses another classful
network. I have extended Bob's scenario and added another router that is R4
and assign 172.16.30.0/24 subnet and R0 on 10.10.10.0/24 subnet.

R0--10.10.10.0/24--R1--172.16.10.0/24--R2--172.16.20.0/24--R3<http://10.10.10.0/24--R1--172.16.10.0/24--R2--172.16.20.0/24--R3>
--172.16.30.0/24--R4

So the above example shows the R1 still sees 172.16.30.0/24 in the routing
table, because it hasn't crossed or passed via another classful network.
But, R0 sees 172.16.0.0/16 as a result of new classful boundary between R0
and R1. Same results for EIGRP and RIP.

The main difference between EIGRP and RIP is explained below:

RIP -- If the auto-summary was disabled on R0, R1 will still summarise
10.10.10.0/24 to 10.0.0.0 when advertising to router R2 and onwards.
EIGRP -- If the auto-summary was disabled on R0, R1 will not summarise the
route 10.10.10.0/24 and advertise as it is

Thanks again for pointing into right direction- if the above explanation
makes any sense:)

Regards,

Bilal

On
 Sun, Sep 5, 2010 at 12:14 AM, Bob Sinclair <bob_at_bobsinclair.net> wrote:

> Hello Bilal,
>
> EIGRP will auto summarize prefixes to their classful boundaries if, and
> only
> if, the router has a connected, internal network in that classful network.
> RIP auto summary will summarize prefixes to their classful boundaries
> whether or not the router has a connected network in the classful prefix.
>
> For example, suppose EIGRP on R1, R2 and R3 in the following network:
>
>
> 10.10.10.0/24--R1--172.16.10.0/24--R2--172.16.20.0/24--R3
>
> If auto summary is enabled on R1 and both networks are internal, R1 will
> auto summarize. R2 and R3 will see 10.0.0.0/8. If the router were
> running
> RIP v2, instead of EIGRP, you would see the same results
>
> If auto summary is enabled on R1 and 10.10.10.0 is redistributed into
> EIGRP,
> R1 will not summarize. R2 and R3 will see 10.10.10/24. If they were
> running RIP v2, instead of EIGRP, R1 would auto summarize; RIP has no clue
> re internal or external.
>
> If auto summary was disabled on R1, but enabled on R2, EIGRP on R2 would
> not
> auto summarize the 10.10.10.0/24 network. RIP would.
>
> Highly recommend you lab this up and check it out. If you see anything
> different, let me know!
>
>
> Bob Sinclair CCIE 10427 CCSI 30427
> CIERS2 Online Instructor
> www.tinyurl.com/ciers2online
>
> > -----Original Message-----
> > From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> > Bilal Hansrod
> > Sent: Saturday, September 04, 2010 9:44 AM
> > To: Cisco certification
> > Subject: Auto Summary Revisited
> >
> > Hello Team,
> >
> > I'm trying to get my head around to understand EIGRP auto-summary. I
> > have
> > read some archive and various material, but unable to understand in
> > detail.
> > What I understood so far is that, EIGRP auto-summary will summarise
> > routes
> > that have same first 8 bit, For example,
> >
> > R1 has multiple loopbacks 10.0.0.0/24, 10.0.1.0/24, 10.1.0.0,
> > 10.2.0.0/24,11.0.0.0/24, 11.1.0.0/24, 12.1.1.0/24, 12.2.2.0./24, so if
> > the
> > auto-summary is enabled, it will create a summary route in it's routing
> > table and point to null interface to as a loop prevention technique.
> > Then it will advertise 10.0.0.0/8, 11.0.0.0/8, 12.0.0.0/8 to neighbour
> > routers. Is this correct understanding, if not can anyone please
> > explain.
> > What will happen if that was RIP with auto summary enabled.
> >
> > Regards,
> >
> > Bilal
> >
> >
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> >
> >
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Received on Sun Sep 05 2010 - 02:24:12 ART