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Question on auto-summarization in routing protocols

1. When auto-summarization is on in EIGRP, I notice it summarizes to the classful boundary (ie: 192.168.3.0/24). 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?

Thanks

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JFrederick29

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ASKER

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

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Don Johnston

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ASKER

Hi Don,
Ok, so the 172.16.16.0/22 is a supernet address correct?

2. Auto summarization (from a routing protocol such as ripv2 or eigrp) would advertise the route as 172.16.0.0/16 correct?

Don Johnston

No. It's a subnet of the 172.16.0.0/16 network.

-Don

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ASKER

ok, thanks man.

what about my other question. WIth auto summarization the route would be advertised as 172.16.0.0/16 by the routing protocols that support VLSM? Right?

JFrederick29

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> 172.16.16.0 255.255.252.0

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ASKER

thanks guys