Networking Blog

Rewriting DSCP (ToS) values is tested on Cisco switch 3750:

Ports  	   	Model  	   	    SW Version  	  SW Image
------ 	  	------ 	  	    ----------------- 	  ------------
52 	        WS-C3750G-48TS      12.2(25)SEE4 	  C3750-ADVIPSERVICESK

By default QoS configuration is disabled:

sw3750#sh mls qos
QoS is disabled
QoS ip packet dscp rewrite is enabled

So we need to enable QoS:

sw3750#conf t
sw3750(config)#mls qos
sw3750(config)#end

sw3750#sh mls qos
QoS is enabled
QoS ip packet dscp rewrite is enabled

We need to create access list:

sw3750#conf t
sw3750(config)#access-list 1 remark rewrite_dscp
sw3750(config)#access-list 1 permit any

sw3750#show access-lists 1
Standard IP access list 1
  10 permit any

Now we’ll begin with real QoS, and use terms as “class map” and “policy map“. According Cisco’s official documentation:

A class map is a mechanism that you use to name a specific traffic flow (or class) and to isolate it from all other traffic. The class map defines the criteria used to match against a specific traffic flow to further classify it. The criteria can include matching the access group defined by the ACL or matching a specific list of DSCP or IP precedence values. If you have more than one type of traffic that you want to classify, you can create another class map and use a different name. After a packet is matched against the
class-map criteria, you further classify it through the use of a policy map.

A policy map specifies which traffic class to act on. Actions can include trusting the CoS, DSCP, or IP precedence values in the traffic class; setting a specific DSCP or IP precedence value in the traffic class; or specifying the traffic bandwidth limitations and the action to take when the traffic is out of profile. Before a policy map can be effective, you must attach it to a port.

sw3750(config)#class-map ?
   WORD class-map name
   match-all Logical-AND all matching statements under this classmap
   match-any Logical-OR all matching statements under this classmap
sw3750(config)#class-map match-all class-map-dscp
sw3750(config-cmap)#match access-group 1

sw3750#show class-map class-map-dscp
 Class Map match-all class-map-dscp (id 1)
   Match access-group 1

After creting class-map, we need policy map. In this policy map we’ll rewrite dscp value of packet: “set dscp af32″ , this is ToS 0×70

sw3750(config)#policy-map vlan101
sw3750(config-pmap)#class class-map-dscp
sw3750(config-pmap-c)#set dscp af32

sw3750#sh policy-map vlan101
 Policy Map vlan101
  Class class-map-dscp
   set dscp af32

OK, we created calss map and policy map, and now we’ll aplly them to interface.

sw3750(config)#int vlan101
sw3750(config-if)#service-policy input vlan101

sw3750#sh ru int vlan101
interface Vlan101
  no ip address
  service-policy input vlan101
end

On a trunk port apply next commands:

sw3750(config)#int gi1/0/4
sw3750(config-if)#mls qos vlan-based

sw3750#sh ru int gi1/0/4
  interface GigabitEthernet1/0/4
  switchport trunk encapsulation dot1q
  switchport trunk allowed 101
  switchport mode trunk
  mls qos vlan-based
end