How DSCP mapping to CAPWAP ?

[Rasika Nayanajith]

I am trying to understand how DSCP mapping works in CAPWAP mode. Going through SRND4.1(enterprise mobility 4.1 design guide). For my lab set up (see below) I have 3502AP & 4402- WLC  connected to 3750 switch.I am trusting dscp at AP connected switch port & trust cos at WLC connected switch port as per recommended settings. WLAN is configured for Platinum QoS profile. I am focusing traffic going from wireless client to wired PC and see how DSCP value changed each point.

As per my understaning, if VoIP call is originating from Laptop 2 (with original DSCP value EF), AP should map into CAPWAP DSCP to EF (as my QoS profile is platinum). But when I see packet captures at (fa1/0/13- AP connected, G1/0/1-WLC connected), outer DSCP value map into AF41.

Also sigalling traffic goes from Laptop2 to CME, original packet marked with DSCP CS3 (as expected for voip signalling), but mapped into AF21 in outer DSCP in the CAPWAP. I do not know why ?

Pls help me to understand this ? Below is the wireshark capture of spanning the port Fa1/0/13

[Stephen Rodriguez]

The AP takes the original mark from the client, and puts that inot the  CAPWAP header from the AP to the WLC(up to the 802.1p value configured in the QoS profile).  When the packet gets to the WLC, it strips the CAPWAP header and the packet plays out to the LAN just like it had come directly from the client.

Two things you can check.  1.) Check the Platinum profile and make sure that the 802.1p tag is set to 6.  2.) Check your QoS profile on the phone system, and make sure that for a softphone they are set to the correct values.

HTH,
Steve

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[Rasika Nayanajith]

Hi Steve,

Thanks for quick response.

1. Yes I have checked the QoS profile values, below is the "show wlan " output confirming that.

Quality of Service............................... Platinum (voice)

Scan Defer Priority.............................. 4,5,6

Scan Defer Time.................................. 100 milliseconds

WMM.............................................. Allowed

WMM UAPSD Compliant Client Support............... Disabled

Media Stream Multicast-direct.................... Enabled

CCX - AironetIe Support.......................... Enabled

CCX - Gratuitous ProbeResponse (GPR)............. Disabled

CCX - Diagnostics Channel Capability............. Disabled

Dot11-Phone Mode (7920).......................... Disabled

Wired Protocol................................... 802.1P (Tag=6)

2. I think softphone DSCP mapping works as expected. that's why inner packet DSCP value is set to EF & CS3 for RTP & call signalling traffic. I do not understand why/how CAPWAP derive outer DSCP values AF41 & AF21 respectively.

[Rasika Nayanajith]

I found the answer to this behaviour,AP add the outer DSCP based on the WMM_UP of the incoming 802.11 frame( and not based on the original packet DSCP). In the softphone scenario RTP traffic marked with priority 5 (video) eventhough I expected it would be priority 6 (voice). So AP would map WMM_VI (or priority 5) to DSCP AF41.

Also for the SCCP signalling traffic originating from softphone will be marked with priority 3 (EE-Excelent Effort). This value will be translated into AF21 at the AP when it add outer DSCP value based on the incoming 802.11 frame WMM UP.

When I analysis the packet capture of the AP operating channel I found this. Here is the capture for RTP traffic originating from the softphone

Here is the capture for SCCP traffic originating from the softphone.

[George Stefanick]

Wmm_up means the QoS on the WLAN or the 802.1p ?

Sent from Cisco Technical Support iPhone App

[Rasika Nayanajith]

Hi George,

As per my understanding 802.11e/WMM user prioraties are set by any WMM capable device before placing the 802.11 frame into wireless media. WMM capable device will put its traffic into one of four different queues(AC_VO, AC_VI, AC_BE & AC_BK). Each Access Category (AC) represtent two user priority traffic.This WMM_UP used to prioratize traffic in wireless media, ie Client to AP & AP to client.To ensure end to end QoS for wireless traffic this WMM_UP value translated into outer DSCP value at the AP & subseqenty converted into 802.1p value for a layer 2 traffic between WLC-Switch port.

For example Network Control(NC) will mark with UP value 7 & Voice (VO) will mark with UP value 6  by the WMM client before put into AC_VO queue and then to the wireless media.  Video(VI) of UP value 5 & Control Load (CL) of UP value 4 will be go into AC_VI queue prior to go into wireless media.

In my scenario (softphone in Dell D630 Laptop) RTP traffic will go to the wireless media with  user priority value of 5 (as video in WMM_UP). But when I checked this with C7921 wireless phone this traffic is go as user priority value of 6 (as voice in WMM_UP)

HTH

[Rasika Nayanajith]

I have summarized what I have learned so far on this topic (DSCP mapping in CAPWAP) & thought it may be useful to someone else.

http://mrncciew.wordpress.com/2012/12/02/understanding-wireless-qos-part-3/

Please keep in mind I am studying for my CCIE-Wireless & information on my blog posts are based on my current level of  understaning.

[George Stefanick]

good stuff... I need to better understand this myself. BTW I own my80211.com. I will link to your blog ..

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"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
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‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."

[Rasika Nayanajith]

Hi George,

I knew you are the owner of "my80211.com" & it is one of the blog I am freqently visiting to learn stuff (as there are very few wireless blogs available in CCIE level). feel free to link my stuff into yours...

Cheers

Rasika