Designing vlan

Unanswered Question
Nov 29th, 2007

What might be suitable vlan size i mean broadcast domain size. Howmany end users in one vlan. What is the basic risk of creting too big broadcast domains.



I have this problem too.
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Jon Marshall Fri, 11/30/2007 - 00:42

Hi Juha

Different people use different guidelines but a commonly accepted size is a class C network ie /24. A lot does depend on the applications you are running on the vlans ie. if they use a lot of broadcasts to function then you may need a smaller size.

We use /25's per vlan where i work which gives 126 useable IP addresses.

The basic risk of too big a broadcast domain is that your clients spend too much time and resources processing broadcast traffic that may or may not be for them.



paul.matthews Fri, 11/30/2007 - 02:08

Just to add a little more t Jn's comment, keeping the subnet size small also helps should you have problems along the lines of code red or SQL slammer. Once a subet is identified as having an infected device, shutting down the VLAN as the first step before locating the infected device has a lower impact on other users - you are less likely to get grief for taking 70 users offline than if you took 400 users out for a single infected device.

Going for smaller subnets also makes the use of L3 links between switches potentially more practical.


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