BSSID
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BSSID

• The 48-bit (6-octet) MAC address of an access point’s radio is known as the basic service set identifier (BSSID) / the MAC address of the radio network interface in an access point.
• Most often the BSSID is the MAC address of the radio network interface.
• Definition of a BSSID address is the layer 2 identifier of each individual BSS.
• If you have two BSSs near each other, and they are both advertising the same SSID, a client station needs to identify the one BSS from the other. In order for clients to roam seamlessly, the APs must advertise the same SSID.
• The client stations, however, still need a unique layer 2 identifier of each AP. The BSSID provides each BSS with a unique identifier, thus the name BSSID.
• As shown in Figure 7.8, the BSSID address is found in the MAC header of most 802.11 wireless frames and is used for identification purposes of the basic service set.
• The BSSID address plays a role in directing 802.11 traffic within the basic service set. This address is also used as a unique layer 2 identifier of the basic service set.
• The BSSID address is needed during the roaming process.
  

Figure 7.8 Basic Service Set Identifier

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.