430 Configuring quality of service

NN47250-500 (Version 03.01)
Figure 30 on page 429 shows the following process:
1A user sends voice traffic from a WMM VoIP phone. The phone marks the CoS field of the packet with
service type 7, indicating that the packet is for high priority (voice) traffic.
2AP A receives the voice packet and classifies the packet by mapping the service type in the 802.11 header
to an internal CoS value. In this example, the service type is 7 and maps to internal CoS 7.
The AP encapsulates the data in an IP tunnel packet, and marks the DSCP value in the tunnel header
based on the internal CoS value. In this example, the AP maps internal CoS 7 to DSCP 56 and marks the
IP tunnel header’s DSCP field with value 56. The AP then sends the packet to the WSS.
3WSS A receives the packet on the IP tunnel connecting the WSS to AP A. The WSS classifies the packet
based on the DSCP value in the IP header of the tunnel packet (in this example, DSCP 56), and maps this
value to an internal CoS value (in this example, 7).
WSS A marks the packet based on the packet’s internal CoS value. In this example, the egress interface is
in a VLAN and has an 802.1Q VLAN tag. Therefore, the WSS marks both the 802.1p value (with 7) and
the tunnel header’s DSCP value (with 56). WSS A sends the packet to WSS B on the IP tunnel that
connects the two switches.
4WSS B receives the packet from the Layer 3 cloud. The packet has an 802.1Q VLAN tag, so the WSS
classifies the packet by mapping its 802.1p value (in this example, 7) to the matching internal CoS value
(also 7). However, because the packet also has a non-zero value in the DSCP field of the tunnel header,
the WSS reclassifies the packet by mapping the DSCP value (56) to an internal CoS value (7) instead.
5WSS B encapsulates the packet in an IP tunnel packet and marks the DSCP value in the tunnel header
based on the packet’s internal CoS value. In this example, the WSS marks the tunnel header with DSCP
56. WSS B sends the packet to AP B on the IP tunnel that connects them.
6AP B receives the packet and does the following:
●Maps the DSCP value in the tunnel header (56) to an internal CoS value (7).
●Marks the packet’s service type based on the internal CoS value (7).
●Places the packet in a forwarding queue (Voice) based on the internal CoS value (7).
In this example, the AP places the packet in the Voice forwarding queue. The Voice queue has
statistically more access to the air than the other queues, so the user’s voice traffic receives priority
treatment.
Note. In this example, the WSS interface with the AP is untagged, so the WSS
does not classify the packet based on its 802.1p value.
Note. An ACL can override a packet’s marking. If a packet matches a permit
ACL mapped to the outbound traffic direction on the AP port, Distributed AP, or
user VLAN, and the ACL sets the CoS value, the tunnel header’s DSCP value is
marked based on the CoS value in the ACL instead.