WLC QoS Part 1 – Upstream

One of the most frequent questions I get is about how WLC QoS actually operates.  It can be confusing, since various options change what happens at different points in the upstream/downstream train.  In this post, I’m going to start with QoS going upstream from a WMM client to the WLC.  In future posts, I’ll look at downstream QoS, at TSPEC, at non-WMM QoS operation, and at rate limiting.  Each of these has enough to deal with by itself!  Also, keep in mind that the settings and results described here are true for WLC 4.2.  While I don’t believe that these have changed with newer code, I have not had a chance to test those.

By default, Wireless Multi-Media (WMM) is allowed on SSIDs.  WMM is a certification based on the IEEE 802.11e standard.  It sets 8 User Priorities, from 0 to 7.  Remember that values 1-2 are lower in priority than 0 in this standard.  Sets of 2 are grouped together into four Access Categories (AC.)  These are Voice – 6-7, Video – 4-5, Best Effort – 0 & 3, and Background – 1-2.

The UP and AC values come into play with the QoS Profiles on the WLC.  Again, this blog is for the upstream direction, from the client to the WLC.  So, each QoS profile defines the highest value that will be used for marking from the incoming UP value to the outer DSCP value from the access point.  In other words, Platinum allows values to be mapped up to the EF value.  So, UP 6 and 7 are mapped to EF.    Then, UP 5 is mapped to AF41, and so on.  For Gold, up to AF41 is allowed.  Silver allows up to the default DSCP value (“0″.)  And, Bronze allows up to CS1.  These values are true whether or not the 802.1p value is set under the QoS Profile.

So, takeaway #1: QoS Profiles affect the upstream DSCP value by setting the maximum value for DSCP.

Takeaway #2: The AP is where is the transition from the 802.11e value to the AVVID DSCP value is performed.

As a side note: one problem that I saw in testing was that UP 1 was matched to DSCP 2 (not CS1 or CS2.)  That causes issues further upstream with queueing, since DSCP 2 is mapped to CoS 0 by default, and not Cos 1.

Next, we move to the incoming LWAPP packet at the WLC.  The outer incoming DSCP value is mapped to the outgoing DSCP and CoS values.  However, it is not a direct translation.  The translation (except for EF) takes the DSCP value and converts it to an equivalent CS value and associated CoS value.  So, AF11 becomes CS1 with CoS 1.  AF31 becomes CS3 with CoS, and so on.  EF is translated to EF, with CoS 5.  This translation is true unless the 802.1p value is set to “None” in the profile.  In that case, all incoming packets are mapped to DSCP and CoS 0.  Also, note that the inner DSCP value (as sent from the client) never comes into play.  The outer DSCP value coming in determines the outer DSCP value going out (with the appropriate mapping as above.)

Takeaway #3: The outer DSCP value coming into the WLC is loosely mapped to the outgoing DSCP and CoS values.

Takeaway #4: The 802.1p value determines the upper DSCP and CoS value outbound upstream.

Another side note: If the DSCP value is changed in between the AP and the WLC (due to DSCP Mutation or due to DSCP-CoS or CoS-DSCP maps,) that will affect the resultant DSCP and CoS value on the non-LWAPP packet, up to the maximum allowed by the 802.1p value.

Side note three: The DSCP 2 discussed previously is actually mapped to CS2 with a CoS of 2.

Finally, only one QoS Profile affects an SSID at a time.  This may seem self evident.  However, it is important to note that changes in the Gold profile (such as setting the 802.1p value to 4 instead of the default of 5) do NOT affect the packets from an SSID with QoS Profile Platinum.  So, if the change was made to Gold, packets with a DSCP value of AF41 incoming on Platinum would be marked with CS4 and CoS 4 outbound.  However, packets on a Gold SSID with a value of AF41 would be remarked to CS3 and CoS 3.

Takeaway #5: Traffic from a specific SSID are only affected by the specified QoS Profile

Next time, we’ll take a look at traffic in the other direction, downstream from the wired client to the wireless client.  Take care.

Jason Boyers - CCIE #26024 (Wireless)
Technical Instructor – IPexpert
jboyers@ipexpert.com

Print Friendly

Tags: Access Point, CCIE, CCIE Wireless, CoS, Downstream, DSCP, QoS, Upstream, WLC