LWAPP- Design - Vlan(wlan) Subnet size

Unanswered Question
May 26th, 2008

Cisco Wireless BU seem to recommend creating small Users Vlan subnet. Since Broadcast transmission can be block, somebody have a good reason to explain why ?

Does some people run with large subnet (4K - 16K) without any problem ?

I understand that if all users are using the same Vlan and that this one goes bad we can loss all the associating Wlan activities. I am looking for others reasons !

I have this problem too.
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Scott Fella Mon, 05/26/2008 - 20:00

In general, it all depends on the protocols you have on the network. I have seen large number of users on a single subnet and performance was not great. In an Lwapp design, you want to keep the wireless on different subnets than the wired users. So.... 4K - 16K wireless users.... you will never see that. What you are looking at seems to be everything in one subnet like how they did it back in the days... all on vlan 1.

Smaller subnet makes life easier, because you can segment the network in a more logical matter. For example... internal secure wireless would be on subnet 1, voice wireless would be on subnet 2, guest wireless on subnet 3, wired users on subnet 4, wired voice on subnet 5.

Breaking it down like this also helps when you need to apply QoS.... there are a lot of other reasons to break down large subnets...

robetrem Tue, 05/27/2008 - 04:39

Yes I thought that wireless vlan will be different of the wire vlans and yes I was thinking having different Vlan for my wireless application (voice, medical, admin, etc...).

Having said that, do I have a performance hit or something else that I should care in using a big Wireless subnet ? My WLC have layer 2 connectivity in the distribution block. Using AP Group to break the subnet in smaller one bring management tasks that I want to eliminate if I don't really need it.

Scott Fella Tue, 05/27/2008 - 04:50

I understand, but like Cisco suggest.... even the subbnet the ap's are put on, they say no more than 40 ap's on a subnet. No more than 20 ap's if you place them on the same subnet as the WLC.

Using AP Groups to break up the subnet to smaller subnets is the best route. What you can do is break it up per floor. Take a look at how many devices (voice, medical, admin, etc) you have.... usually keeping the subnet at a /24 or /23 should be large enough.

On a L2 approach to the access layer, your subnets will reside on the core where your wlc will be connected to or a distribution switch. If you have WCS, you can create a template and push that template out to a bunch of ap's at one time.

I prefer not to have a large subnet due to my earlier experiences I have had.... Just my opinion.

robetrem Thu, 05/29/2008 - 06:27

Breaking subnet in AP group by floor: chance to roam a lot ? roaming between APGroup will created L3 roaming ? Even if station IP address is kept, I think the load is higher than L2 Roaming. Also I think that Multicast will be break !? Not sure with new version of WLC.

Scott Fella Thu, 05/29/2008 - 08:40

What you want to do is if you have two wlc's split up the upper and lower floors between the 2 wlc's. Example... floor 1-3 on wlc1 floor 4-5 on wlc2. This way you don't have roaming between both controllers. Not that it makes things worse, it just best practice. I have used AP Groups in large roll outs with no issues.

Remember that all traffic is tunneled back to the WLC where you set the multicast and that is where your traffic will get dumped.

dominic.caron Thu, 05/29/2008 - 10:22


My wireless network is design with /21 wireless subnet to facilitate roaming. I dont have any performance problem. My load is usualy around 3000 users.


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