Tuesday, 16 April 2013

Cisco WLC Broadcast and Muticast

Broadcast and multicast traffic often require special treatment within a WLAN network because of the additional load placed on WLAN. In order for all STAs associated to an AP to receive broadcast and multicast traffic, it needs to be sent at the lowest common bitrate.

The default behavior of the WLC is to block broadcast and multicast traffic from being sent out the WLAN to other STAs. The WLC can do this without impacting client operation because most IP clients do not send broadcast/multicast traffic for any reason other than to obtain network information (DHCP) and resolve IP addresses to MAC addresses (ARP).


The WLC acts as a DHCP relay agent for associated WLAN clients. It unicasts client DHCP requests to locally configured or upstream DHCP server except during L3 client roaming. DHCP server IP address(es) are configured for each dynamic interface, which is associated with one or more WLANs. DHCP relay requests are forwarded via the dynamic interfaces using the source IP address of a given dynamic interface. Because the WLC knows which DHCP server to use for a given interface/WLAN, there is no need to broadcast client DHCP requests out its wired and wireless interfaces.


Before a WLAN client can send IP packets to any other IP address, it needs to know the MAC address of the target client to forward the frame to. To accomplish this a client will broadcast an ARP query, equesting the MAC address for the IP host that it wishes to communicate with.

When a WLC sees a wireless client ARP request, it will either respond directly, acting as an ARP proxy in behalf of the wireless client, or it will forward the request out it's wired interface to have it resolved by another WLC. The WLC will not forward the ARP broadcast back out to the WLAN.

Multicast can be explicitly enabled on a WLC, but this requires some steps to be taken to minimize the multicast traffic generated on those interfaces that the WLC connects to. When multicast is enabled, it is a global setting, which means it will be enabled for every WLAN configured regardless if multicast is needed by the WLAN or not.


Cisco Enterprise Mobility 4.1 Design Guide


  1. You say "the default behavior of the WLC is to block broadcast and multicast traffic from being sent out the WLAN to other STAs."

    Does this mean that the WLC is blocking broadcast/multicast from the wireless side from going into the wired side? Does it also work the way other around? Will it block broadcast/multicast from the wired side from going out to the wireless devices?

    Thank you!

  2. The WLC blocks broadcast/multicast traffic when sent from the STA to either another station or on to the wired side. It also blocks (by default) broadcast/multicast traffic from wired network on to the wireless medium, it can be configured to allow this traffic, though.