Dynamic multicast optimization

Dynamic multicast optimization (DMO) is a HPE Aruba Networking innovation that optimizes the forwarding of IP multicast groups using administratively scoped IP multicast addresses. When DMO is enabled and active on a WLAN and radio, explicitly joined IP multicast groups are converted to unicast and transmitted to each IP multicast receiver at their unicast transmit data rates:

1. Permits IP multicast groups to be received by specific wireless stations that explicitly join the IP multicast groups versus all the wireless stations associated with the BSSID.
2. Provides a reliable transmission of IP multicast frames by permitting the AP to retransmit missing unicast frames.

DMO relies on IGMP and MLD snooping on the APs to track IP multicast group membership for each wireless station. When an IP multicast group is joined by a wireless station, the AP will replace the multicast receiver address in the 802.11 wireless header for each multicast frame with the receiving hosts MAC address. The multicast group is transmitted by the transmitting radio at each receiver’s unicast data rate. One copy of each IP multicast frame is transmitted for each wireless receiver. When DMO is enabled for a WLAN, it is either in an active or inactive state based on thresholds defined as part of the DMO configuration within the WLAN profile. When enabled and active, each source IP multicast group is forwarded to each receiver at their unicast data transmit rate. For each IP multicast frame received, the AP will replicate and forward the frame to each receiver of that IP multicast group. When the specified channel utilization threshold or client threshold is reached, DMO is deactivated on the transmitting radio. Each active IP multicast group is then forwarded normally as a 802.11 broadcast multicast frame at either the lowest configurated rate for the radio or if MTO is enabled, the configured MTO rate.

Recomendation

DMO is a unique feature designed to support IP multicast applications and should only be enabled after a design consultation with your HPE Aruba Networking account team. Supporting IP multicast in a wireless environment requires additional LAN configuration and if tunnel forwarding is utilized a cluster design review. Enabling DMO in a WLAN profile also influences how other IP multicast frames are filtered and forwarded by the AP. When DMO is enabled, multicast DNS (MDNS) and simple service discovery protocol (SSDP) frames are forwarded irrespective of the broadcast filtering option applied to the WLAN. Each MDNS/SSDP multicast frame is also converted to unicast prior to forwarding to each active wireless station adding additional load to the APs. If DMO is enabled and MDNS/SSDP needs to be filtered, either AirGroup must be enabled or policies dropping MDNS/SSDP must be applied to the user roles.

Configuration

DMO is enabled and its associated thresholds configured within each WLAN profile. Once DMO is enabled, additional options are displayed within the WLAN profile allowing the channel utilization and client thresholds to be defined. Specific recommendations for threshold values are unique to each environment and are out of scope for this guide. In classic Central, DMO is configured within each WLAN profile under Broadcast/Multicast and on Central, DMO is configured within each WLAN profile under Broadcast/Multicast

Before and after

The following packet capture depicts an IP multicast video stream received by a wireless station when DMO is disabled or inactive. The IP multicast video stream is transmitted at the lowest configured transmit data rate, in this example 24 Mbps:

The following packet capture depicts an IP multicast video stream received by a wireless station when DMO is enabled and active. The receiver address is replaced with the IP multicast receivers host MAC address, and the video steam is transmitted at the receivers unicast transmit data rate: