QOS - Best Practice for LAN Topology (3560 Switches)

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Sep 18th, 2010
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Looking to implement best practice for IPT.

The LAN is a layer 2 network and the core switches are performing interVLAN routing.

What QOS markings should I apply for the below ports. Confused if to apply COS or DSCP for the below.

### Ports ###

Voice Gateway - mls qos trust cos or dscp

Callmanager Cluster (Sub & Pub) - mls qos trust cos or dscp

Uplink circuits (802.1Q) - mls qos trust cos or dscp

Cisco IP Phone Ports - auto-qos voip cisco-phones

EtherChannels (802.1Q) - mls qos trust cos or dscp

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Overall Rating: 5 (1 ratings)
Lei Tian Sat, 09/18/2010 - 10:25
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If all the devices have DSCP value marked correctly, you can trust DSCP on all ports.


Lei Tian

colmgrier Sat, 09/18/2010 - 10:55
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I applied 'mls qos trust devide cisco-phone' under port G0/1. This marks the packets as COS??

interface GigabitEthernet0/1
description "PC & IP Phone"
switchport access vlan 10
switchport mode access

switchport voice vlan 100

srr-queue bandwidth share 10 10 60 20

queue-set 2
priority-queue out
mls qos trust device cisco-phone
mls qos trust cos
auto qos voip cisco-phone
spanning-tree portfast
spanning-tree bpduguard enable
service-policy input AutoQoS-Police-CiscoPhone



Jon Marshall Sat, 09/18/2010 - 11:14
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"mls qos trust cos" does not mark the packets with a CoS value, rather it uses the existing CoS value in the 802.1q tag and prioritzes the packet based on that value.

As Lei says if the phone can mark DSCP as well then it is a good idea to trust DSCP all the way through your network as there will be no need to convert back and forward between CoS and DSCP.


Lei Tian Sat, 09/18/2010 - 12:00
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Hi Colm,

As Jon said 'mls qos trust cos' doesn't mark the packet.

In addition, trust cos can sometime cause problem. For example, a incoming frame with COS =5 DSCP = EF, the port configured as trust cos. With default setting, when packet leaves the switch, it will have COS = 5 DSCP =40. That's because the default cos-dscp maps cos=5 to dscp=40, and the default behavior of the switch is dscp rewrite.


Lei Tian

lorenz84gDD Fri, 11/26/2010 - 10:13
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I'm implementing QoS on a LAN similar to your LAB.

What kind of policy did you use between your core switches and IPT block switches.

I believe you configured the link between these devices as L3 routed.

I already trust COS, so if you have two SVI do you need to map cos to dscp?

did you find a solution for this?




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