Sizing number of SFP ports on the WLC

[johnruffing]

Any comments on determining how many SFP ports should be utilized on a 4404? The hard limit appears to be no more the 48 APs are allowed per SFP port on the WLC. More pertinent, has anyone loaded up 48 APs per port on the WLC and what has been the result in terms of performance?

In the Cisco WCS/WLC class, the rule-of-thumb given was to light up one SFP for every 24 access points.

Some of the factors that I am thinking of that would impact this number are that we are intending to run only 802.11b/g (no 802.11a). That should cut the expected bandwidth coming from the APs in half (almost, not accounting for LWAP control traffic).

Does anyone happen know what the LWAP control traffic bandwidth is (between each AP and WLC) - or at least a rule of thumb to use?

Again, if you have run 48 APs on a single port, have there been any performance issues? Were you running both 802.11a and b/g?

Thanks.

[smalkeric]

The hardware on the 4400 data ports is not configurable, so it will go always at 1000 full duplex, even if using a 1000BaseT SFP.

[johnruffing]

Unfortunately, your answer does not address my question.

What I am trying to determine is HOW MANY ports need to be utilized for a given number of access points when configured for 802.11b/g only (no 802.11a).

[Darren Ramsey · Accepted solution]

Like you said, 48 AP's is the hard limit per gig port on the 4400 series. I had 48 AP's on one port for a month or two, while waiting for a downtime to reboot and enable LAG. There were no issues. I looked back at the Gig interface usage for the 4400, and the peak I've seen is 20MB. There are around 110 b/g clients and 60 AP on that controller.

[jakew]

There is an imposed limitation of 48 APs per port on any 440x platform to mitigate against oversubscription based on an assumption of ~20MB per AP. Obviously, dual-mode Cisco APs are capable of handling over double that in good RF environments. So take your environment, applications, and what kind of tolerance for oversubscription you have into account.

To scale beyond 48 ports, you have 2 options:

1. Use LAG on the controller

2. Use multiple AP Manager interfaces (each assigned to a different port)

I usually recommend you go with option 1 if possible, if only because it's simpler. The WLC handles the load-balancing of traffic across the ports dynamically.

For (2), keep in mind that LWAPP will dynamically load balance the APs across the AP managers. So be careful because you're still statically mapping your dynamic WLAN interfaces to a port on the WLC. The traffic has to go in and go out of the controller so you could potentially oversubscribe a port.