WLC QoS – 802.1p Profile & WMM Disabled

I’ve received a few emails regarding my previous posts, plus some questions during the CCIE Wireless bootcamp last week.  Namely, the questions/statements were the following:

1. Your statement about the 802.1p value only applying to the WLC itself and not to the AP wired correction is incorrect
2. You did not include anything about WMM Disabled, so the conclusions could be incorrect for that scenario

Let me address each one separately, starting with the 802.1p value.  First, the value does indeed affect the wired traffic passing through the WLC.  So, if a frame is received with CoS 5, but the max 802.1p value is 3, the frame will be remarked to CoS 3 when the WLC puts it on the wire.  Also, if the 802.1p type is None, then the frame will be remarked to CoS 0.

On the access point, the value for 802.1p CAN have an impact.  If the 802.1p type is set to None, then the wired packet from the AP will not be affected.  However, if it is set, it will be the maximum “internal” UP value allowed on the AP.  Here are some scenarios to explain it more.

A client connects to an SSID with WMM Allowed.  The QoS Profile is Platinum, but the 802.1p value is set to 5. The A client sends an 802.11 frame upstream marked with UP 6 (or with DSCP EF if not a WMM device).  That is received by the AP.  Since the max is 5, the outgoing packet is marked with AF41 instead of the normal DSCP value of EF for either the WMM or the non-WMM device.  Remember that the AP will mark down the DSCP value so that it corresponds with the Cisco AVVID requirements.  So, UP 6 was marked down to UP 5, which was used to mark DSCP AF41.  Downstream is similar.  A packet with DSCP EF is received by the AP.  It would be mapped to UP 6.  However, with the max of 5, it is marked with UP 5 and placed in queue 5 to be sent to the client.

Takeaway #1:  The 802.1p value CAN affect both the wired AP and wired WLC connections.

The second question is also important.  What happens to my packets when WMM is Disabled for an SSID?  It can be very confusing, especially when Cisco states, “Non-WMM clients have the DSCP of their LWAPP tunnel set to match the default QoS profile for that WLAN.”  We have already shown that this is not correct.  What would be a better statement is this, “All clients on an SSID with WMM Disabled have the DSCP of their LWAPP tunnel set to match the default QoS profile for that WLAN, depending on the 802.1p value.”  In other words, if the SSID is set to use the Platinum QoS Profile, which is set to have a maximum 802.1p value of 6, and the SSID has WMM Disabled, then all traffic received from clients will be marked with DSCP EF.  If the 802.1p value is changed to 5, then all client traffic will be marked with DSCP AF41.

Takeaway #2: All clients on an SSID with WMM Disabled have the DSCP of their LWAPP tunnel set to match the default QoS profile for that WLAN, depending on the 802.1p value.

I thought that the last one was worth mentioning again.  Hopefully this cleared up some confusion.  Next time we’ll look at CAC/ACM/TSPEC and its impact on WLC QoS.

Jason Boyers – CCIE #26024 (Wireless)
Technical Instructor – IPexpert, Inc.
Mailto: jboyers@ipexpert.com

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Tags: CCIE, CCIE Wireless, CCIE Wireless Training, LWAPP, QoS, quality of service, Voice, WLC, wmm