So far we have looked at how a packet’s QoS label (DSCP/CoS/WMM_UP) changing at different points on its journey to wireless side to wired & vice versa. I have used Platinum QoS settings on WLC & WMM settings as “Allowed”. In this post we will look at how these different QoS profiles (Platinum, Gold, Silver & Bronze) affecting packets QoS labels on its way.(to make it little bit simpler I will look at traffic from wireless client to wired pc only).
This is more or less logical representation in a way to explain these mappings. Physically AP & WLC connected to the same switch(Fa1/0/13- Access Port & G1/0/1 -Trunk port ). I have used C7921 phone as WMM capable client to generate traffic (RTP & SCCP). Also I have used ICMP traffic originated from PC->7921 to generate another type of traffic flow in my network.That traffic is classified as CS2 when it enters fa0/1 in my CME router. You will see ICMP responses going from 7921->PC as 3rd type of traffic in this analysis. If you capture the wireless traffic on the channel where HQ-AP01 is operating & filter the traffic from C7921 phone you would see something like this.(Only RTP traffic capture is shown here, but all the mapping values were taken in these diagram as per wireshark captures even though I have not attached all those captures here )
WMM_UP in the 802.11 frame is set by the WMM client prior to send a 802.11 frame into wireless media. There are 8 UP values defined by WMM/802.11e standard. These 8 UP traffic will map(two UP into one AC) into one of 4 different queues (AC_VO, AC-VI, AC_BE & AC_BK) in order to prioritize traffic in wireless media.In our example RTP traffic will mark of priority 6(Voice), SCCP with Priority 4(Control Load) & ICMP echo-responses with priority 0 (Best Effort). Also show the original packets DSCP value as I observed.(these value depend on how client classify its traffic based on its wireless driver- I noticed softphoe & C7921 phone RTP traffic marks with two different priority 5 & 6 in 802.11 frame).
Now when these packets go to AP it will do the CAPWAP encapsulation and adding outer DSCP value based on the WMM_UP received on the incoming 802.11 frame.When this packet entered into switch (at Fa1/0/13 in my example) it will trust the outer DSCP value & rewrite 802.1p before sending it out to WLC connected trunk port (G1/0/1). Following shows the values I observed for my 3 types of traffic. 1st table show the capture values at Fa1/0/13 & 2nd table shows the capture values at G1/0/1 for the traffic going from AP to WLC.
Note that WMM_UP 6 map into DSCP EF, WMM_UP 4 map into DSCP AF31, WMM_UP 0 map into DSCP 0. This is irrespective to whatever the original packet DSCP it carries (EF,0 & CS2 in my case). At the AP connected switch port (Fa1/0/13) this outer DSCP will be trusted & derive 802.1p/CoS values for the L2 frame sending out to WLC (G1/0/1). This is depend on how you configure “mls qos map dscp-cos” mapping on your switch. With default config DSCP values mapping would be EF(46)->5, AF31(26)->3 & 0->0.
Now when traffic goes from WLC to PC via G1/0/1 switch port will trust 802.1p values (as it is configured to trust CoS) & rewrite packet DSCP values based on cos-dscp mapping configured. Therefore when it received by the PC original packet DSCP get altered. Below shows these QoS values observed at G1/0/1 & Fa1/0/4. Note that CoS values 5 , 3 & 0 map into DSCP EF, CS3 & 00 respectively which is different to original packets DSCP (EF, CS0 & CS2). 1st table shows the values captured at G1/0/1 & 2nd table show the values captured at Fa1/0/4 for the traffic going from WLC to PC.
Now we will look at if QoS profile on the WLC is configured for Gold instead of Platinum. If it is Gold profile, WMM_UP priority 5 equivalent DSCP would be the maximum settings of outer DSCP can be set by AP/WLC. Therefore EF traffic will be mark-down as AF41 in the outer DSCP. Below tables shown QoS values that I observed when my WLC QoS profile set to Gold & same 3 traffic type go from wireless client to wired PC. First two tables show mapping respect to AP->WLC & the second two tables show mapping respect to WLC->PC traffic.
You would noticed that if WLC QoS profile is Gold, then outer DSCP will be capped to AF41 for incoming traffic which has WMM_UP 6 or above. So when EF traffic received by wired PC as CS4(no longer remain as EF).
If you configure the QoS profile as Silver, when it translate WMM_UP to outer DSCP it will capped to WMM_UP 3 as max. This is equivalent to AF21 in the mapping table (see part 1 of this series for more info). This is how you should see these value get alter across its traffic path when QoS profile is silver.
In Summary, if you want to preserve DSCP values upto EF, you have to configure the WLAN QoS profile as platinum. If you configure Gold, Silver (default profile) & Bronze original packet DSCP will be capped to CS4, CS2 & CS0.
In the next post, will see what will happen if client is non-WMM capable/WLC configured for WMM scenario. Also how packets QoS parameters vary if WMM is disabled on WLC. I know it’s going to be few more posts require to cover thing off, but I have to dig it deep to get required understanding for my CCIE wireless exam on this topic.
1. Understanding Wireless QoS – Part 1
2. Understanding Wireless QoS – Part 2
3. Understanding Wireless QoS – Part 4
4. Understanding Wireless QoS – Part 5
5. 3750/3560/2960 Wired QoS
6. Who do you trust ? (DSCP or CoS)