Cisco QoS Cookbook for LAN

Cisco provides an abundance of documentation about Quality of Service. This Wiki intends to boil down the information to a quick and simple LAN switch configuration guide, that works in most customer environments.

What to do

• Check the used CoS and DSCP values of the products used. Most Siemens products will use the DSCP values
  • AF31 (i.e decimal value 26, binary 011010) and 802.1p (CoS) value 3 for voice signaling and
  • EF (i.e decimal value 46, binary 101110) and 802.1p (CoS) value for voice streams,

If in doubt, configure the phones/gateway/media server etc. to use those values.

• Turn on auto-qos on Cisco LAN switches and routers. This will turn on QoS in Cisco's switches and routers and will configure it with values, that match most customer's LAN environments. E.g. CoS=5 and DSCP=EF will be sent to the priority queue, if the router/switch is capable of doing so. Packets found in the priority queue are sent before any other packets are sent and therefore are used for real-time traffic.

• Define Trust boundaries: you need to talk to your customer on whether it is acceptable for him to trust its employees about the proper usage of packet priority markings.
  • If the customer trusts on proper use of packet markings, and the access switches support this, configure mls qos trust dscp on access ports. Else configure mls qos trust cos.
  • If the customer needs to control the proper usage of packet markings, the LAN switch needs to classify the packets based on other means. For Cisco routers, the most secure and convenient way is to use Cisco NBAR for classification at the network edges. For Cisco IOS LAN switches, you need to use VLAN access-lists instead. ((( To be completed ...)))

A Real Life Example with trust-cos

Catalyst 6500

Here is a real customer configuration example:

version 12.1
# switch on QoS globally:
mls qos
...
interface Port-channel1
 description Channel-Port
 no ip address
 mls qos trust cos
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 storm-control broadcast level 5.00

interface GigabitEthernet2/1
 description This is a one port of a channel with VLAN trunk
 no ip address
 logging event link-status
 wrr-queue cos-map 1 2 2 
 wrr-queue cos-map 2 1 4 
 # DSCP=EF is mapped to CoS=5; IEEE recommends CoS=6 for voice, and CoS=3 is used for signaling
 # all packets with 802.1p-values (CoS-values) 3, 5 or 6 are sent to the priority queue 1
 priority-queue cos-map 1 3 5 6 
 # the current module has ingress queues. This makes sense, since the Cat6500 is not non-blocking.
 rcv-queue cos-map 1 2 2 
 rcv-queue cos-map 1 3 4 
 mls qos trust cos
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 storm-control broadcast level 5.00
 channel-group 1 mode desirable non-silent

Catalyst 3550

interface FastEthernet0/2
 description this is an access-port
 switchport access vlan 2
 switchport mode access
 no ip address
 mls qos trust cos
 storm-control broadcast level 5.00
 power inline never
 wrr-queue cos-map 1 0 1
 wrr-queue cos-map 2 2 4
 wrr-queue cos-map 3 7
 wrr-queue cos-map 4 3 5 6 # all packets with 802.1p-values (CoS-values) 3, 5 or 6 are sent to the priority queue (4)
 priority-queue out        # switch on priority-queuing. On Cat3550, this turns queue number 4 to a priority queue
 no cdp enable

See also

Quality of Service