Controller Based WLANs

When should I enable "drop-mcast" in a SSID profile?

by on ‎07-02-2014 09:04 AM

Product and Software: This article applies to all Aruba controllers and ArubaOS 3.3.1.5 and later.

 

When you enable "drop-mcast" in a SSID profile, Aruba prevents all broadcast/multicast frames from going out to the air on the SSID, so the wireless client does not need to turn on its Rx radio module for every DTIM. DTIMs normally happen quite often, so this results in less power consumption.  Also, multicast is transmitted at the lowest transmit rate possible, so the overall bandwidth available for unicast transmissions is greatly increased.

 

Enabling "drop-mcast" could be useful in any the following environments:

 

  • a large number of clients all on the same broadcast domain
  • clients that emit DHCP renewals on every roam (so, there's relatively large amount of broadcast frames seen all the time)
  • the battery capacity of the clients are very limited

These environments are mostly VoWLAN deployments.

 

Note these two very important caveats before you enable "drop-mcast":

 

  •  If any of your clients are using any sort of broadcast-based protocol (for example, Windows sharing), this traffic will automatically be dropped by the controller. Note that this configuration parameter is available per virtual AP and so any multicast applications should be moved to another virtual AP.
  •  You have to use this command with "voip-proxy-arp" enabled. If you do not enable "voip-proxy-arp", no communication can occur. This happens because ARP uses broadcast traffic to communicate, and that traffic will be dropped if you enable "drop-mcast". Enabling "voip-proxy-arp" allows the clients to communicate.

 

Lastly, you need the voice license for this command. You also need the voice license for the firewall voip-proxy-arp command.

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