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This is the continuation from my previous post (Understanding Wireless QoS – Part 1) about Wireless QoS. So far we have looked at packets going from C7921 phone to wired PC-softphone. Few key points are,

1. Configured “mls qos trust dscp” on Fa1/0/13 – AP connected switch port.
2. Configured “mls qos trust cos” on G1/0/1 – WLC connected trunk port.
3. RTP traffic from C7921 will be encapsulated by the AP to WLC.
4. Outer DSCP header added by AP based on the incoming 802.11 frame priority (WMM_UP) & QoS profile configured on the WLAN.
5. Only Outer DSCP header is visible to switch port & switch will rewrite DSCP/CoS values in this outer header before sending out to a switch port (Original packet DSCP intact-no change)

Now will look at DSCP/CoS changes on the return traffic (PC->C7921). I have taken TCP signalling (Skinny -SCCP) coming from CME & RTP traffic coming from softphone on the PC for analysis. If  you take a capture at fa1/0/4 you would see something like this. CME classify SCCP traffic with DSCP-AF31. Softphone classify its RTP traffic as DSCP-EF.

When you look how these packets DSCP valued get change when it goes to WLC, you would see something like this. You noticed switch will rewrite both EF & AF31 in to DSCP 0. This is because we have not trusted these at Fa 1/0/4. Once you configured “mls qos trust dscp” on fa1/0/4 you will see packets going to WLC will have the original DSCP (AF31 & EF respectively). Switch will add corresponding CoS value to 802.1q header as port is configured as Trunk.when it comes packet path from WLC to AP, CAPWAP encapsulation comes into play, an outer DSCP value corresponding to original packet DSCP will be added by WLC(simply copy inner DSCP to outer DSCP, but cap to a max as per QoS profile configured ). In my example QoS profile is configured as Platinum, it is simply copy the original DSCP value into outer DSCP (No cap). Packet capture of the trunk port will show this.

Wireless QoS-2-6

Wireless QoS-2-5

Now when you look at the packets going towards at Fa1/0/13 ,you will notice that SCCP traffic outer DSCP changed to CS3. This is because at the WLC connected port we trust CoS value of 3 (corresponding to DSCP-AF31- see the mapping table in the previous post) & when exiting fa1/0/13 it will re-write DSCP value derived from cos-dscp mapping. But this will have no impact on the original packet DSCP value.

CAT2#sh mls qos map cos-dscp
Cos-dscp map:
cos:   0  1  2  3  4  5  6  7
dscp:   0  8 16 24 32 46 48 56

So when this packet goes into wireless media from the AP to the wireless phone it these will map into 802.11e (WMM) user priorities. This mean RTP (outer DSCP EF) map into 802.11e/WMM UP 6 & Signalling traffic (DSCP CS3) map into 802.11e/WMM UP 4.

In the next post we will see how other QoS profiles (Gold , Silver & Bronze) impacting these QoS settings.

Related Posts

1. Understanding Wireless QoS – Part 1
2. Understanding Wireless QoS – Part 3
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)