Controller Based WLANs

How does spectrum load balancing work and how do I configure and troubleshoot it?

Product and Software: This article applies to all Aruba controllers and ArubaOS 3.0 and later.

Why do we need spectrum load balancing?

AP-based load balancing was widely used in customer deployments, but it had the following drawbacks:

  • Required probes to be handled centrally, which needed to turn off "local-probe-response" under SSID profile, and was not very scalable in large deployments.
  • Had static values for client thresholds which made it difficult to adapt its behavior if the client population changed dynamically (for example, flash crowds in classrooms and conference events).
  • Did not consider the fact that balancing spectrum utilization rather than number of clients per AP will result in reduced co-channel interference and hence better performance.


How Does It Work?
Spectrum load balancing (SLB) is based on the concept of creating virtual RF neighborhoods among access points for efficient client association management:

  • The virtual RF neighborhoods are automatically created using ARM scanning, client density updates, and neighbor exchanges
  • This ensures APs that are far away from the clients perspective do not belong to the virtual RF neighborhood, to avoid redirecting clients to a far away AP that is on a lightly loaded channel
  • APs in the virtual RF neighborhood participate in client load balancing using association responses to direct clients towards APs on less loaded channels
  • If there are 20% fewer clients on other channel(s) as compared to the AP's channel, then we enable SLB on the AP

Note: Certain deployments prefer to have ARM scanning and assignment disabled, because AP positions, channels, and powers have been carefully planned. The feature has an additional extension to allow manual creation of virtual RF neighborhoods using the parameter "spectrum-load-bal-domain" in the rf dot11a(g)-radio-profiles. This is not a recommended mode of operation.


How Do I Configure SLB?
Use these commands:

configure terminal rf dot11a(g)-radio-profile XYZ spectrum-load-balancing

ARM scanning must be enabled using the rf arm-profile.

configure terminal rf arm-profile XYZ scanning

For manual creation of virtual RF neighborhoods:
configure terminal rf dot11a(g)-radio-profile XYZ spectrum-load-bal-domain "ABC"

  • If SLB is enabled and SLB-domain is empty, controllers use automatic virtual RF neighborhood calculations as described earlier.
  • If SLB is enabled and SLB-domain is defined, radios that belong to the same SLB-domain are considered for load balancing. In this case, the controller ignores ARM-based RF neighborhoods or any client density inputs.
  • Note that this setting should be used only in deployments where ARM assignment and scanning have to be disabled in favor of static channel and power allocations and is not a recommended configuration.


CLI Commands for Monitoring

(Aruba800) #show ap active

Active AP Table
---------------
Name Group IP Address 11g Clients 11g Ch/EIRP/MaxEIRP 11a Clients 11a Ch/EIRP/MaxEIRP AP Type Flags Uptime Outer IP
---- ----- ---------- ----------- ------------------- ----------- ------------------- ------- ----- ------ --------
palo-4 palomino 10.3.136.248 0 3 AP:HT:36+/15/21:L 125 A 11d:21h:52m:33s N/A
palo-1 palomino 10.3.136.252 0 2 AP:HT:157+/15/21 125 A 11d:21h:56m:50s N/A

L: flag indicates that load balancing is enabled.

(Aruba800) #show ap arm state ap-name palo-1

AP-palo-1:10.3.136.252:157:15:21-Edge:enable : Client Density:4
Neighbor Data
-------------
Name IP Address SNR Assignment Neighbor Density
---- ---------- --- ---------- ----------------
palo-4 10.3.136.248 66 36/15 3/3/75/100

Client Density: 4 = Number of unique clients palo-1 can see
3/3/75/100 = Number of clients common between palo-1 and palo-4 /
number of unique clients palo-4 can see
Ratio of overlapping clients to palo-1's seen clients
Ratio of overlapping clients to palo-4's seen clients


Both ratios have to be greater than 50% for APs palo-1 and palo-4 to be virtual RF neighbors.

Therefore APs palo-1 and palo-4 are load balancing neighbors and will try to distribute clients amongst them.


For advanced troubleshooting, the following command can be used:

show log user with user logging enabled for the client MAC address

Sep 23 08:33:40 10.1.1.80 2008 [10.1.1.80] stm[1602]: <501095> <NOTI> |stm| Assoc request: 00:1f:3b:8c:27:b5: AP 10.100.139.237-00:1a:1e:87:b1:31-Test-AP125
Sep 23 08:33:40 10.1.1.80 2008 [10.1.1.80] stm[1602]: <501101> <NOTI> |stm| Assoc failure: 00:1f:3b:8c:27:b5: AP 10.100.139.237-
00:1a:1e:87:b1:31-Test-AP125 Reason AP is resource constrained


(AP Test rejects association because load balancing is enabled.)

Version history
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Last update:
‎07-05-2014 03:10 AM
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