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When to subnet to reduce broadcast domain

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Oct 2nd, 2003
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Is there a rule of thumb for the number of nodes on one subnet before you need to break it up? I know this depends on many factors, but given an average office environment with no particularly chatty devices, is there a threshold at which point you can expect a degradation in service?

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milan.kulik Thu, 10/02/2003 - 03:32
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Designing Cisco Networks by Diane Teare (the official CCDA coursebook), page 119 says these maximal worksation numbers per a broadcast domain:

IP - 500

IPX - 300

AppleTalk - 200

NetBIOS - 200

Mixed - 200



d.higginbotham Thu, 10/02/2003 - 04:51
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Thanks Milan, this is exactly the number I was looking for and the reference is especially helpful in that I can quickly obtain further info.

Thanks much,


mftaylj Thu, 10/02/2003 - 07:33
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A much better indicator is when you approach 20% of the segments traffic being Broadcast / Multicast traffic it is time.


d.higginbotham Thu, 10/02/2003 - 07:52
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That sounds like a number the boss might be interested in. What's the best way to determine what percentage of the traffic is broadcast/multicast?

mftaylj Thu, 10/02/2003 - 08:10
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I use a Sniffer once the interface stats for the vlan in question start looking bad.

I also have broadcast supression configured on my uplink ports set at 20%.


d.higginbotham Thu, 10/02/2003 - 08:30
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Please excuse my ignorance, but what stats are you looking at? sh port x/x and look at collisions?

mftaylj Thu, 10/02/2003 - 09:03
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On a CatOS based switches - "show mac" will give you the numbers you are after.


d.higginbotham Thu, 10/02/2003 - 10:26
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Thanks again Jim, the link is much appreciated. I don't anticipate any problmes but I like to be prepared.


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