Configured QoS

Configured QoS
To configure QoS on a 3550 switch, first enable QoS globally with the mls qos command.
The use of class-maps and policy-maps to define the match and classification criteria for
incoming traffic is very similar to the way they are used inside the 2950.
Class-maps can be configured using an extension allowing the matching of either all or
any of the criteria specified in the map. To manage this feature, use either the class-map
match-all or the class-map match-any global commands. In addition, the class-map
supports matching against a VLAN or a group of up to 30 VLANs. To select this match
option, use the match vlan vlan-list c-map command.
The following example shows traffic arriving at interface gigabitethernet0/1, sourced from
VLAN 66 or being already marked with an IP Precedence of 1, having the DSCP set to 63 at
the ingress:
Terry_3550(config)#mls qos
Terry_3550(config)#access-list 101 permit ip any any precedence 1
Terry_3550(config)#class-map match any terry2

Terry_3550(config-cmap)#match access-group 101
Terry_3550(config-cmap)#match vlan 66
Terry_3550(config-cmap)#exit
Terry_3550(config)#policy-map ip_or_VLAN66
Terry_3550(config-pmap)#class terry2
Terry_3550(config-pmap-c)#set ip dscp 63
Terry_3550(config-pmap-c)#exit
Terry_3550(config-pmap)#exit
Terry_3550(config)#interface gigabitethernet0/1
Terry_3550(config-if)#service-policy input ip_or_VLAN66
If you are configuring QoS inside a trusted domain and you do not use auto-QoS, then you have
to decide what to do about trust. If you wish to trust incoming CoS values, use the interface command
mls qos trust cos to ensure that the CoS value in received traffic is trusted, and use the mls
qos trust device cisco-phone command to specify that the Cisco IP phone is a trusted device
and ensure that a non-trusted device does not misuse the CoS available. Remember to enable CDP:
Terry_3550(config)#int fa0/1
Terry_3550(config-if)#cdp enable
Terry_3550(config-if)#mls qos trust ?
cos Classify by packet COS
device trusted device class
dscp Classify by packet DSCP
ip-precedence Classify by packet IP precedence
<cr>
Terry_3550(config-if)# mls qos trust cos
Terry_3550(config-if)# mls qos trust device cisco-phone
Terry_3550(config-if)#^c
Because trusted traffic will automatically gain access to the process whereby CoS is mapped to
DSCP, there is an option to forward CoS values without changing the existing DSCP (and vice
versa) through the switch. This is called pass-through and can be configured for either option:
Terry_3550(config-if)# mls qos trust cos pass-through dscp
or
Terry_3550(config-if)# mls qos trust dscp pass-through cos