FortiWLC – Load Balancing for APs in Virtual Cell

Load Balancing for APs in Virtual Cell

You can configure load balancing to effectively distribute wireless clients to alternate access points. The load balancing is performed by the controller based on two factors; Current Load of the AP and RSSI value of the client.

  • Current load of an AP – Current load represents the number of clients assigned to an AP. Load Balancing for APs in Virtual Cell
  • RSSI value of the Client – The RSSI value of the client is received by the controller.

When a new client joins the network, the controller will connect the client to an AP that is running below its maximum load threshold and providing the best RSSI value.

To enable load balancing, configure the Load Threshold for the access point. Go to Configuration > Wireless > Load Balance.

  1. Load Balancing vCell: Select On to activate this functionality.
  2. Load Threshold: Specify the load threshold. This value denotes the number (in percentage) of clients that can connect to an AP. Example, if the optimum capacity of an AP is 80 clients, and the threshold is set to 90%, then a maximum of 72 clients are allowed to connect.
  3. RSSI Threshold- Configurable via CLI (load‐balance‐vcell rssi‐threshold <rssivalue>). Specify the RSSI value of the best and an alternate AP. Load balance is activated for a value below the configured RSSI value. The default value is -65dbm and the configurable range is -75dbm to -45dbm. The following table provides the recommended RSSI threshold for various modes and channel bandwidth:
  20 MHz 40 MHz 80 MHz 160 Mhz / 80+80 Mhz
802.11b -76 dbm NA NA NA
802.11a/g -65 dbm NA NA NA

Load Balancing for APs in Virtual Cell

  20 MHz 40 MHz 80 MHz 160 Mhz / 80+80 Mhz
802.11n -64 dbm -61 dbm -58 dbm NA
802.11ac -57 dbm -54 dbm -51 dbm -48 dbm

nPlus1 Support: The load balance feature allows the clients to connect to the best available access point during roaming in an nplus1 set up.

The following table illustrates various load balancing scenarios between two APs (AP1 and AP2) and the expected result when a client tries to join the network. :

  • L1 represents the load on AP1; L2 represents the load on AP2. The value ‘1’ represents AP1 has reached its load threshold.
  • R1 represents RSSI value on AP1, and R2 represents RSSI value of AP2, The value ‘1’ represents an RSSI value that is higher than the configured value.
Scenario Expected Result
L1=1, L2=0 and R1=0 and R2=0 Since AP1 is running in full capacity the client will be assigned to AP2.
•  L1=0,  L2=0 and R1=0 ,R2=0

•  L1=0 , L2=0 and R1=-1, R1=-1

•  L1=1, L2=1 and R1=1 and R2=1

In these scenarios, the controller will use default association mechanism to assign the client to AP.
•        L1=0, L2=1 and R1=1, R2=0 L1=1, L2=0 and R1=1, R2=0 L1=1, L2=0 and R1=1, R2=1

•        L1=1, L2=1 and R1=1, R2=0

In these scenarios, the client will be assigned to AP2.
For other cases where L1 or L2 =1 The client stay associated with the current AP i.e. AP1

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