!-RE: Auto Summary Revisited

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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Received on Sat Sep 04 2010 - 10:14:53 ART