Question on auto-summarization in routing protocols

1.  When auto-summarization is on in EIGRP, I notice it summarizes to the classful boundary (ie:  Is this just how IGP's work with auto-summarization on?  They will always summarize to a classful boundary correct?  /8  /16  /24 ?

If I turn off auto-summarization ,  I notice I see the REAL subnet mask. (thanks donjohnston for this info)

2.  Is auto-summarization usually used in production enviornments?  What if you have an internal network that spans several sites.  Each router is running EIGRP. Would you use auto summarization there?  I imagine yes if the networks are contiguous?

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1.  Yes, it summarizes to the classful boundary.

2.  I never use auto-summary.  I always disable it and summarize manually.  This gives you the control over which routes are summarized.

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dissolvedAuthor Commented:
When you say summarize manually...what do you mean?

Do you mean you just diable auto-summarization and let the routing protocol advertise the real mask.
Or is there an actual "manual process" that lets you summarize manually?

Thanks J
Don JohnstonInstructorCommented:
It's manually process.

If you have a number of networks on one side of your router like

You could manually summerize these as

With auto summerization, the router would advertise

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dissolvedAuthor Commented:
Hi Don,
Ok, so the is a supernet address correct?

2. Auto summarization (from a routing protocol such as ripv2 or eigrp) would advertise the route as correct?
Don JohnstonInstructorCommented:
No. It's a subnet of the network.

dissolvedAuthor Commented:
ok, thanks man.

what about my other question.  WIth auto summarization the route would be advertised as by the routing protocols that support VLSM? Right?
Yes, that is correct.  With auto summarization enabled (default), EIGRP or RIPv2 is going to advertise the route using the classful mask, which, could lead to problems if using discontiguous subnets.

To manually summarize using EIGRP:

interface s0/0
ip summary-address eigrp <as>
dissolvedAuthor Commented:
thanks guys
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