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Small Broadcast Domains


I'm attempting to understand some basic design rules for enterprise wireless networks, one of them seems to suggest the layer 2 broadcast domain (VLAN) need to be kept small.  However I don't seem to be able to quite find out the reasons why.

I have read broadcast and multicast traffic have an affect, can anyone elaborate on this and explain exactly how this type of traffic affects a wireless vlan and slows it down

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It is a multi-faceted problem, to be sure. Here are the main concerns as I see them.

Bandwidth on the LAN, though plentiful, is still a resource to be managed and conserved. The more hosts you have in a broadcast domain, the more broadcast traffic your network will have. Cisco recommends 500 ethernet hosts per broadcast domain as a maximum to keep broadcasts from impinging on the available network bandwidth.

Specifically in the world of wireless devices, you want to be even more judicious because bandwidth is (typically) less than your hard-wired LAN. Also more unnecessary traffic means shorter battery life for your wireless devices.

The primary examples of broadcast traffic are ARP (Address Resolution Protocol), and DHCP (Dynamic Host Configuration Protocol), and network switches. When any device wants to send data to another device, it needs the IP (layer 3) and MAC (layer 2) address. If the MAC is unknown, and ARP broadcast goes out to all the devices on the subnet. As the number of hosts grows, these ARP broadcasts can start to noticeably affect network bandwidth. DHCP broadcasts are similarly sent to all devices when a computer comes online and requests an address. Switches, though designed to cut down broadcast traffic, are still going to rebroadcast any broadcast to all devices if it has not yet learned (cached) the location of the destination MAC.

As such, keeping your wireless broadcast domain small (e.g. in its own VLAN) may significantly cut down on the number of broadcasts that are retransmitted wirelessly. This reduction in traffic would be even more pronounced if the wireless IPs are sharing a network with wired devices. Limiting the number of wireless devices per broadcast domain makes sense from that perspective.

Hope this helps!
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Jakob Digranes
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Cisco recommend that, in high-density client environments, the subnet size is actually somewhere around /22 (1022 hosts).

If you use a controller it should proxy broadcast traffic, converting it to unicast by default, so other clients on the network shouldn't see broadcast traffic unless you specifically configure that behaviour.

Multicast can also be converted to unicast, so again, only the relevant clients ever see that traffic.