Tuesday, April 20, 2010

QoS Example LAN Switch Configuration (from Enterprise Mobility 4.1 Design Guide)

QoS Example LAN Switch Configuration

AP Switch Configuration

The QoS configuration of the AP switch is relatively trivial because the switch must trust the DSCP of the LWAPP packets that are passed to it from the AP. There is no CoS marking on the LWAPP frames coming from the AP. The following is an example of this configuration. Note that this configuration addresses only the classification, and that queueing commands may be added, depending on local QoS policy.

interface GigabitEthernet1/0/1
switchport access vlan 100
switchport mode access
mls qos trust dscp
spanning-tree portfast

In trusting the AP DSCP values, the access switch is simply trusting the policy set for that AP by the WLC. The maximum DSCP value assigned to client traffic is based on the QoS policy applied to the WLANs on that AP.

WLC Switch Configuration

The QoS classification decision at the WLC-connected switch is a bit more complicated than at the AP-connected switch, because the choice can be to either trust the DSCP or the CoS of traffic coming from the WLC. In this decision there are a number of points to consider:
Traffic leaving the WLC can be either upstream (to the WLC or network) or downstream (the AP and WLAN client). The downstream traffic is LWAPP encapsulated, and the upstream traffic is from AP and WLAN clients, either LWAPP encapsulated or decapsulated WLAN client traffic, leaving the WLC.
DSCP values of LWAPP packets are controlled by the QoS policies on the WLC; the DSCP values set on the WLAN client traffic encapsulated by the LWAPP tunnel header has not been altered from those set by the WLAN client.
CoS values of frames leaving the WLC are set by the WLC QoS policies, regardless of whether they are upstream, downstream, encapsulated, or decapsulated.
The following example chooses to trust the CoS of settings of the WLC, because this allows a central location for the management of WLAN QoS, rather than having to manage the WLC configuration and an additional policy at the WLC switch connection. Other customers wishing to have a more precise degree of control may wish to implement QoS classification policies on the WLAN-client VLANs.

interface GigabitEthernet1/0/13
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 11-13,60,61
switchport mode trunk
mls qos trust cos


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