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21 mask network on wireless


I currently have 4 controllers with multiple vlans configure for one ssid.  The vlans are group by building and each vlan is a /24 network.   I have been asked to create one vlan per controller with a /21 network.  I am worried about the size of this network and impact it will have on my wireless users.   Do I have an issue with possible packet collision in wireless?  Also I currently don’t allow wireless users, via the check box on the controller, to talk to each other will this stop the broadcast issues with such a large network? 

3 Replies 3


Hi Steve

I do know that there is a University in the USA using /21 on their wireless:

950 APs
Peaks of 1000-1300 concurrent users
/21 network is assigned
About 50-60 packets per second of Broadcast


We are talking about different kind of broadcasts/collission:

RF: Each AP talks at a certain channel to all associated clients. Only one can talk at the same time. Thats why it´s not a good idea to have too many clients per AP.

IP: An IP-network is a IP broadcast-domain. That means that clients that sends broadcasts (l2 or l3) talks to all other devices within the same broadcast-domain. However, when talking about Cisco WLC-solution the IP-network size is irrellevant as long as the wlc is configured the right way. You can configure the system to not allow client to client-traffic as well as not allowing broadcasts. That´s why you can have thousands of clients in the same IP-network when using Ciscos leightweight wireless solution.

Br Jimmy

I currently have the peer to peer blocking set to drop so this stops client to client right?  Does this block the broadcast or is there something else I need to configure like broadcast forwarding?

Note:  I am running version layer 3 mode.

thanks for your help

Steve Chapman

LAN Engineer

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