ARP unicast conversion

When broadcast filtering in a WLAN profile is configured for ARP (recommended) or Unicast ARP only, the APs provide additional optimization by converting ARP requests destined to wireless stations that originate from the LAN or overlay tunnel to unicast.

When an ARP request is converted to unicast, the destination broadcast address is replaced with the recipients host MAC address and is transmitted at the recipients unicast data rate versus the minimum configured transmit data rate configured in the WLAN profile. This provides several benefits:

1. The ARP requests are only transmitted to specific recipients versus all the stations associated with the BSSID.
2. The ARP requests are transmitted at higher transmit data rates vs. the lowest configured transmit data rate conserving airtime.

Additionally, converting ARP requests to unicast ensures the recipient station receives the request as unicast frames are acknowledged by the recipient. If the ARP request is lost in transmission, the AP can retry and retransmit the ARP request frame.

Before and after

The following packet capture depicts a broadcast ARP request for a wireless station that is forwarded at the minimum transmit data rate. The receiver’s destination address in the 802 11 header is a broadcast address (ff:ff:ff:ff:ff:ff) and the ARP request is received by all wireless stations associated with the BSSID irrespective of their assigned VLAN:

The following packet capture depicts an ARP request for a wireless station that has been converted to unicast and is forwarded to the recipient station at its unicast data rate. The reciever address in the 802.11 frame header is replaced with the recipients host MAC address and the ARP request is only received by the recipient: