If you’ve worked with Wi-Fi (2.4/5.0GHz), it’s not recommended to create too many SSIDs. This is because it increases the management overhead (which uses up valuable airtime for Beaconing) and can reduce client data transmission throughput. However, with the introduction of 6 GHz, they’ve tackled this problem by allowing you to advertise multiple SSIDs using a single beacon frame. This capability is provided by an Information Element called “Multiple BSSID” within a beacon frame. The diagram below summarizes this concept:

In this post, we will explore the MBSSID feature. We used the same test topology given below.

I have simply defined 9 SSIDs (mrn-psk to mrn-psk9) on my 9800WLC & advetise it via the Policy Tag “pt1-mrn-lab”. Simply copy & paste below CLI where [X]=2 to 9
wlan wp[X]-psk 10[X] mrn-psk[X] radio policy dot11 6ghz no security ft adaptive no security wpa wpa2 security wpa psk set-key ascii 0 Cisco123 no security wpa akm dot1x security wpa akm sae security wpa akm sae pwe h2e security wpa wpa3 security pmf mandatory no shutdown ! wireless tag policy pt1-mrn-lab description "Policy Tag 1" wlan wp1-psk policy MRNH-Flex wlan wp2-psk policy MRNH-Flex wlan wp3-psk policy MRNH-Flex wlan wp4-psk policy MRNH-Flex wlan wp5-psk policy MRNH-Flex wlan wp6-psk policy MRNH-Flex wlan wp7-psk policy MRNH-Flex wlan wp8-psk policy MRNH-Flex wlan wp9-psk policy MRNH-Flex
Now with those settings, I have taken a PCAP (airtool_mbp_C9166_9mbssid_1.pcapng) on CH53 [80MHz]- Cisco 9166 AP operating channel, you can see how AP advertised these SSIDs within a single beacon frame. Unlike in the legacy bands (2.4/5GHz), where you see a beacon for each SSID, in the 6GHz you only see a single Beacon frame. This frame includes a ‘Multiple BSSID‘ element to advertise the capabilities of those SSIDs.

The BSSID carries the Multiple BSSID element known as the “Transmitted BSSID or Reference BSSID,” and the BSSID carried by MBSSID is known as the “non-transmitted BSSID.”
You noticed that C9166 only advertised 8 SSIDs in 6GHz. A quick check on the 9166 Deployment Guide confirms it supports 8 SSIDs on 6GHz radio on 17.7.x – 17.9.x code version. It looks like 17.12.x (which I used in my 9800 WLC) still has the same limits in place.
“In Release 17.7.1, 17.8.1, and 17.9.1, you are limited to eight Wi-Fi 6E SSIDs. However, this limit will be increased to the standard 16 SSIDs in a future release This limitation affects only 6 GHz; for legacy bands, you can configure up to 16 SSIDs.“
AP advertises its support for MBSSID in Beacons and Probe Responses. Client STA advertises its MBSSID support in Probe Request and association Request frames. It will be under “Extended Capabilities” (Octet 3) as shown below. Support for the Multiple BSSID capability is mandatory for a FILS STA and non-AP HE STA.

Multiple BSSID element include “Nontransmitted BSSID Profile” subelement which includes. It also has max BSSID indicator (3 in my case specifies max 8 BSSID support- 2^3).
- Non Transmitted BSSID capability
- SSID Parameter set
- Multiple BSSID index
- Vendor Specific elements

Individual BSSID is derived from reference BSSID value (Below is taken from IEEE-802.11ax-2021)

I have connected my Pixel6 client to those SSIDs to see the BSSID for individual SSIDs. You can see my reference BSSID (BSSID for the Beacon frame for “mrn-psk” SSID) value 8c:1e:80:e5:62:67. So A5=67[hex], n=3 (max BSSID indicator)

If you look at the BSSID Index 1 (mrn-psk8), you can derive the BSSID for it as 8c:1e:80:e5:62:60
A5(1) = A5 - B + ((B + 1) mod 2^n) A5(1) = 67 (hex) - 7 + ((7 + 1) mod 2^3) A5(1) = 67 (hex) - 7 + (8 mod 8) A5(1) = 67 (hex) - 7 + 0 A5(1) = 60 (hex)
You verify BSSID Index 1 corresponds to “mrn-psk8” by looking at the 1st Nontransmitted BSSID Profile in the MBSSID element.

Even AP advertises 8 SSIDs, as we use MBSSID support, management overhead is not that much. You can check channel utilization reported by AP ( in the QBSS element if the vendor supports it) or using Spectrum Analyzer (L1 perspective). In the given capture you can see 1-3% reported in the QBSS element (Note due to low data rate <24Mbps disabled, you will see lower channel utilization in QBSS). Out of the total frames, 10.2% are Beacons within that capture.

The below graph shows the I/O statistics (In Wireshark, Statistics > I/O Graphs) comparing Beacon frames count against total frames. Overall around 10% are beacon frames (10 beacons per every 1s approx). Note that with the MBSSID element, the beacon size is around 800 bytes in the above.

To compare, I have enabled WLAN for 5GHz and put my Cisco 9166AP into CH165 with all those 9 SSIDs broadcasting. I have taken a PCAP (airtool_mbp_C9166_CH165_9mbssid.pcapng) while doing the same test (connecting my Pixel6 client to those SSIDs). As you can see in the PCAP, 60% of the frames are Beacon frames (90 Beacons for every 1s approx).

References
1. Multiple BSSID – Improve Airtime Efficiency by Adrian Granados
2. Cisco 9162I, 9164I & 9166I Deployment Guide