switchport QoS configuration for WLC

[welleslee]

Hi guys,

I am having a question about switchport QoS configuration for WLC. In the "enterprise mobility deployment guide 4.1", it says we can use both either "mls qos trust cos" or "mls qos trust dscp", depending on the mapping, however looking through Jerome's video tutorial on QoS in youtube, he actually mentioned that we need to keep this value as "mls qos trust cos", and I have also read from somewhere else that since controllers are layer 2 device, we do need to use "mls qos trust cos" on the switchport directly connected to the WLC.

however I did see some configurations using "mls qos trust dscp" on the switchport, which is for 5508 WLC due to some hardware design in the 5508 WLC. so I just want to ask what is the differnece between these two commands when we use them for switchports connected to WLC, and does this command really specific for what controller platform we use or it only depends on code version?

thank you very much for your time and help.

[George Stefanick]

Andy,

Great question and one that comes up from time to time. Here is my undersetanding ...

MLS QOS TRUST COS comes into play when you use trunks. So, on the switch ports connecting to the WLC, COS should be used.

MLLS QOS TRUST DSCP comes into play when you use layer 3. So, at the access point, the port connecting to the ap DSCP should be used. The only time this is different is when HREAP is used with a trunk at the swithc port, then COS should be used.

Keep in mind, your qos profile should be properly configured.

I hope this helps .

[welleslee]

Hi George,

Thanks for your reply, however I am still a bit confused on the difference between the two commands...

so in the deployment guide it does say we can do either one on the switchports connected to the WLC, so when should we use trust dscp instead of cos?

it sort of make sense when we talk about WLC is a layer 2 device hence we should use cos, but we also have packets marked with dscp values and I think the controller will keep the value without any modification, then that means we can also use trust dscp in order for this mapping to be done correctly, is this correct?

I did see some other people saying that when using turst dscp and did a packet capture, the values cannot be mapped correctly, but there are also cases that this works. expecially with 5508 series controllers, does it really have some hardware features that doing different things than the 4404s and the 2100 series?

but still, in what situation should we trust dscp on switchports connecting to WLC and when should we use trust cos as a best practice? or we should just forget about trust dscp for switchport connecting to WLC?

Thanks.

[George Stefanick]

Andy,

The best way to get a comfort level with this is to wireshark it and take a peek.

If it is an access port, there are no CoS values in the frame and hence you can’t trust CoS.  CoS values are specified in the Prioritization part of the VLAN tag in the 802.1Q header.  This also means that there are no CoS values in untagged native VLAN frames. SInce we truck at the ports leading into the controller we use COS.

[George Stefanick]

I also wanted to add:

A colleague of mine just passed the CCIE wireless lab..His comments:

"based on the incoming DSCP value the WLC maps it to a COS value and the ensures the value does not exceed the upper limit according to the WLAN QoS profile assigned. "