Command of the Day

Aruba Employee

COTD: Spectrum Load Balancing

Why do we need spectrum load balancing?

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

  • Required probes to be handled centrally, which 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 (flash crowds in classrooms, conference events etc.)
  • 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 is based on the concept of creating virtual RF neighborhoods amongst access points for efficient client association management

  • The virtual RF neighborhoods are automatically created via ARM scanning, client density updates and neighbor exchanges
  • This ensures that 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

Note: Certain deployments prefer to have ARM scanning and assignment disabled since 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 to configure SLB?

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

ARM scanning needs to be enabled via 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 spectrum-load-balancing (SLB) is enabled and SLB-domain is empty, controllers use automatic virtual RF neighborhood calculations as described earlier.
  • If spectrum-load-balancing is enabled and SLB-domain is defined, radios belonging to the same SLB-domain are considered for load balancing. In this case, the controller will ignore ARM-based RF neighborhood 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/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 0 3 AP:HT:36+/15/21:L 125 A 11d:21h:52m:33s N/A
palo-1 palomino 0 2 AP:HT:157+/15/21 125 A 11d:21h:56m:50s N/A

L: flag indicates load balancing is enabled

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

AP-palo-1: : Client Density:4
Neighbor Data
Name IP Address SNR Assignment Neighbor Density
---- ---------- --- ---------- ----------------
palo-4 66 36/15 3/3/75/100

Client Density: 4 = No. of unique clients palo-1 can see
(3/3/75/100 = No. of clients common between palo-1 and palo-4 /
No. 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 commands can be used

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

Sep 23 08:33:40 2008 stm: <501095> |stm| Assoc request: 00:1f:3b:8c:27:b5: AP
Sep 23 08:33:40 2008 stm: <501101> |stm| Assoc failure: 00:1f:3b:8c:27:b5: AP
00:1a:1e:87:b1:31-1323-AP125 Reason AP is resource constrained
(AP 1323 rejects association since load balancing is enabled)

Re: COTD: Spectrum Load Balancing

Thank you for giving the thresholds for when APs are determined to be RF neighbors. Can you please also state what the thresholds are for an AP to have spectrum load balancing "activated" (i.e., the "L" flag)? In other words, what is the ratio need to be for an AP's client count versus one of its neighbors in order for SLB to take effect?
Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
Aruba Employee

Re: COTD: Spectrum Load Balancing

If there are 20% fewer clients on other channel(s) as compared to the AP's channel, then we enable SLB on the AP
Occasional Contributor II

Re: COTD: Spectrum Load Balancing

Thanks, Sachin!

I got a quick question, would you mind clarifing why AP would reject association with a reason being "Reason AP is resource constrained"? Is it because the AP had a limit on how many client can associate to this particular AP?

Re: COTD: Spectrum Load Balancing

Meesick, I believe I can answer this for you.

The way SLB works is that once an AP has identified that other APs (on other channels) are less utilized, the SLB-enabled AP will send an association failure of "Resource constrained". This is an effort to encourage the cient to try and associate to another AP. It's not that the AP is actually unable to service other clients; it's that it is trying to influence the client.

The SLB-enabled AP will only fail the association once in a row (from what I've been told). If the client sent a subsequent association request to the same SLB-enabled AP, it would be allowed to associate.
Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
Occasional Contributor II

Re: COTD: Spectrum Load Balancing

Sweet! Thanks, Ryan!
Frequent Contributor I

Re: COTD: Spectrum Load Balancing

Good info, thanks.
How does the Max. Association on the SSID Profile affect SLB? Does ARM take this first into consideration or it ignores it when SLB is enabled?
Marcelo Lew
Wireless Network Architect-Engineer
University of Denver
Guru Elite

MAX Association

MAX association is the hard limit, regardless.

Colin Joseph
Aruba Customer Engineering

Looking for an Answer? Search the Community Knowledge Base Here: Community Knowledge Base


SLB in practice - questions

We have a room with five APs in it. I previously had SLB enabled with ARM, but the area was surrounded by other APs, and neighbor relationships were established that were not intended. Because we wanted to simply load balance among the five APs in the room, I disabled ARM, established a static channel/power plan, and enabled SLB with a specified SLB domain. The five APs are as follows:

Active AP Table
Name Group IP Address 11g Clients 11g Ch/EIRP/MaxEIRP 11a Clients 11a Ch/EIRP/MaxEIRP AP Type Flags Uptime
---- ----- ---------- ----------- ------------------- ----------- ------------------- ------- ----- ------
thompson-2-24 tnc3-thompson-balance 8 AP:1/14.5/33:L 3 AP:149/19/36:L 70 E 151d:22h:59m:6s
thompson-2-25 tnc3-thompson-balance 8 AP:6/14.5/33:L 1 AP:157/18.5/36:L 70 E 151d:22h:50m:35s
thompson-2-26 tnc3-thompson-balance 3 AP:11/14.5/33 2 AP:161/18.5/36:L 70 E 151d:22h:59m:56s
thompson-2-27 tnc3-thompson-balance 0 AP:1/14.5/33:L 2 AP:153/18.5/36:L 70 E 151d:22h:30m:5s
thompson-2-28 tnc3-thompson-balance 11 AP:6/14.5/33:L 5 AP:48/18.5/23:L 70 E 151d:22h:59m:36s

One request for affirmation and one question:
1.) AP "thompson-2-27" has zero 11g clients but has SLB enabled. I'm guessing this is because the other AP on channel 1, "thompson-2-24" has numerous clients, and since they're on the same channel, SLB is applied to all APs in that domain on that channel?
2.) All five APs' 11a radios have SLB enabled. What is the benefit of that? The result is that all five APs would deny the first association request. Because there are only five in the domain, I would expect that at least one of them would not have SLB enabled. Explain.
Ryan Holland, ACDX #1 ACMX #1
The Ohio State University
Occasional Contributor II

Re: COTD: Spectrum Load Balancing

Hi all,
I have a question about Spectrum Load Balancing.
If user had Bandwith Load Balancing request.
That "Spectrum Load Balancing" can set about "Bandwith"?
Like in conference room, have two APs(ap-1 and ap-2).
If a client use large-bandwith to download on ap-1, other client will associate to ap-2. (I rember Aruba3.3 have the function and work fine.)

Can "Spectrum Load Balancing" do this function?
I never set "Spectrum Load Balancing", so I do not know "Spectrum Load Balancing" is conform user request or not.

Please help, thanks.
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