Size of a VLAN

Answered Question
May 30th, 2008

Hi,

We will be implementing a VLAN - with the primary goal being to limit broadcast traffic. My question is - how large should each individual VLAN be? Most of the workstations would be Windows XP (around 80%), some Vista, some Windows 95/98 still around.

Will a /24 (for example 172.16.x.x/24) for each VLAN be ok? Or should I make it larger or smaller? The Windows workstations will be accessing the Internet, and local fileservers, local Oracle/SAP servers.

Thanks,

Tony

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Correct Answer by andrew.butterworth about 8 years 7 months ago

The rule of thumb I usually work to is if the network consists of entirely IP hosts (most of the time...) then go no bigger than a /23 (i.e. max 510 hosts), if you have other protocols running in addition to IP then go no higher than a /24 (254 hosts). However with this in mind you need to consider the network topology, STP diameters, fault footprint/isolation and redundancy.

It is generally recommended to keep VLAN's local to a switch, i.e. don't span VLANs between access switches. Doing it this way keeps fault STP simple and loop-free, it keeps potential fault footprints isolated (broadcast storms etc), allows easy troubleshooting (you know where your IP subnets are physically) and is easy to grow due to it's modularity.

Have a read of the High Availability Campus Design SRND - http://www.cisco.com/go/srnd

HTH

Andy

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Mark Yeates Fri, 05/30/2008 - 20:10

Tony,

A /24 VLAN is a good size for a VLAN on a network. Since you are using private IP addressing and don't have a need to conserve IP addresses which can be helpful for future expansion. You can make them as big or small as you would like based on your requirements. It is easier to have your VLAN's segmented to things such as different departments so that you can filter traffic accordingly if necessary. You just don't want too many PC's (500+) on the same VLAN as it may be more difficult to troubleshoot individual problems.

Mark

Correct Answer
andrew.butterworth Sat, 05/31/2008 - 03:22

The rule of thumb I usually work to is if the network consists of entirely IP hosts (most of the time...) then go no bigger than a /23 (i.e. max 510 hosts), if you have other protocols running in addition to IP then go no higher than a /24 (254 hosts). However with this in mind you need to consider the network topology, STP diameters, fault footprint/isolation and redundancy.

It is generally recommended to keep VLAN's local to a switch, i.e. don't span VLANs between access switches. Doing it this way keeps fault STP simple and loop-free, it keeps potential fault footprints isolated (broadcast storms etc), allows easy troubleshooting (you know where your IP subnets are physically) and is easy to grow due to it's modularity.

Have a read of the High Availability Campus Design SRND - http://www.cisco.com/go/srnd

HTH

Andy

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