Making sure QoS does its job as expected includes ensuring that the WLC is properly configured to translate between WMM markings on the WLAN and CoS/DSCP markings on the LAN. Many of us know that the WLC uses the Platinum QoS profile which is associated to the Access Category (AC) AC_VO which uses a user priority (UP) marking of 6. On the wired LAN, we tend to use a layer 2 CoS (or 802.1P) marking of 5, which is also usually mapped to a layer 3 DSCP marking of 46, or EF. So we know that a WLC or AP should map the UP 6 to a CoS 5 when bridging frames between the WLAN and LAN.
I’ve been reviewing some past projects, and I was reminded that Cisco documentation used to recommend that we map UP 6 to CoS 6. This didn’t make a lot of sense to me at the time, especially when I wanted the voice traffic to end up with a CoS 5 or DSCP EF on the wired LAN. Here is a screenshot I had taken from the Cisco configuration guide at the time:
Sometimes mistakes make their way into documentation, so at the time I decided I had better test to see what the WLC would do with these settings. I set up packet captures on the wired switch ports and used a Cisco 7925 handset to get WLAN captures. With the QoS profile mapping set according to the screenshot above, here is what happened:
- The 7925 marked the voice payload frame with UP 6 at L2, and DSCP EF at L3
- From AP to WLC, the CAPWAP encapsulated frame was marked with DSCP EF
- APs were on access ports, so no L2 CoS markings
- When the WLC placed the packet on the wired LAN as an 802.3 frame, the markings were L2 CoS 5 and L3 DSCP EF
Wait – didn’t we map the Platinum (UP 6) profile to CoS 6? Even better, if we mapped the profile to Cos 5, the result was… exactly the same!
Turns out Cisco’s was saying one thing and doing another. Here’s a recent tech-note where they come clean:
I knew I remembered this from somewhere! What a odd behaviour. Note that the all that has changed is the default setting and thus the recommendation – we should map WMM 6 to CoS 5. The behaviour hasn’t changed and if you map to CoS 6, well then Cisco still goes ahead and spits out a 5 anyhow.
And frankly, that’s the right choice in my opinion.