mribbans_FTNT
Description
A common issue with Fortinet Virtual Cell technology  is if an Access Point is misconfigured, then it results in Wireless clients performing a ‘hard hand off’, when roaming between AP’s.

This article explains how to find a misconfigured AP radio to ensure seamless roaming by ensuring all radios in the ESS profile are broadcasting the same BSSID.


Solution
When an ESS (Extended Service Set) Profile is created on a Fortinet Wireless Controller and the virtualization mode is set to ‘Virtual Cell’ all Access Point (AP) radios which are part of the virtual cell will broadcast a common BSSID ( per band, i.e. 2.4GHz or 5GHz).
This common BSSID enables the Fortinet Wireless Controller (and not the client) to determine when a unit is moved between access points; from the Wireless client’s perspective it is still connected to the same AP.
In order for a common BSSID to be broadcast however, certain radio parameters for each Access Point in the ESS Profile need to be set identically.
This includes: the radio channel, the channel width, the RF Band and the MIMO mode.

If any of these radio parameters are different, then the AP radio in the same Virtual Cell will broadcast a different BSSID, thus resulting in a hard handoff when the unit roams which can results in Wi-Fi drop outs and jitter.
An easy way to find an AP radio that is misconfigured (or to audit the Virtual Cell ESS profiles) is to use the WLC’s command line interface to display a list of all BSSID, while filtering it for a particular ESS profile.
This is possible using the command: show ess-ap in conjunction with the ESS profile name listed after a pipe ‘ | ’ symbol.

This way filter the results is possible and see a complete list of all the BSSID’s, just for a particular ESS Profile.
Any AP radio that has a different BSSID will have at least one of the aforementioned parameters set differently to the others.

The command to use is:
#  show ess-ap | <ESS_profile_here>>
In the example below, an ESS Profile called Voice_5G is filtered  to see where the broken virtual cell is.

In the output that the 5GHz radio (Interface Index 2) on AP ID 176 has a different channel width configured, and was therefore broadcasting a different ESSID as highlighted.

Any voice client that roamed to this Access Point have noticed some jitter or drop outs on the call.
# show ess-ap | Voice_5G                               
ESS Profile    APID    AP Name    If Index    Channel    Operating Channel    Admin State    Max Calls    BSSID
Voice_5G        176    AP-176 Level 3    2       149              149             Up    0    00:0c:e6:02:b6:d5
Voice_5G        166    AP159 Level  2    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        174    AP-174 Plant A    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        175    AP-175 Plant B    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G         10    AP-13 Hall F      2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        103    AP-45 Hall G      2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        104    AP-81 Level 5     2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        105    AP- 82 Level 3    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        106    AP-113 Level 2    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        107    AP-112 Near to    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        110    AP-92 Halls L     2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        112    AP-94 Hall H      2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        115    AP-111 Hall J     2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        116    AP-110 Foyer      2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        117    AP-104 Level 6    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        118    AP-121 Stage 2    2       36                36             Up    0    00:0c:e6:02:82:4a
Voice_5G        119    AP-78 Stage 3     2       36                36             Up    0    00:0c:e6:02:82:4a

If a Broken Virtual Cell is found:

If after running the above command, an AP in a Virtual cell is broadcasting a different BSSID, find it, edit that AP radio and see what differs from the rest of the Access Points.
To do this, from the Forti-WLC’s web GUI select the following:

1) Go to  Configuration -> Wireless -> Radio  to open the relevant section.
2) enter the AP ID number and  AP Index (1 is typically 2.4GHz & 2 is 5GHz radio) in the boxes along the top and then enter to filter the table and highlight the affected AP radio
3) Select 'edit' (pencil icon), as shown below.



In the next section, check each of the highlighted items (Channel, Channel Width, RF Band and MIMO Mode) and compare to any other AP radio which is broadcasting the a common BSSID.
At least one will be different.


Note: Changing radio parameter will result in the Access Point rebooting.





To verify, use the following command:
# show ess-ap | <ESS_profile_here>
The Access Point is now broadcasting the same BSSID

Note: If the AP is not broadcasting the SSID it is because  ???? .... check these steps ????

If there is a mixture of Access Point models?

Even if there is a mixture of AP models, a Virtual Cell can be created in certain situations by editing the AP radios as shown above.
Refer to the compatibility matrix below.





Related Articles

Meru Technical Note - AP command to display the BSSIDs of the radio for each ESS profile

Meru Technical Note - What is Shared BSSID and Per-station BSSID?

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