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arvindai (TechnicalUser) (OP)
1 May 08 19:47
I need to configure QOS on my cisco 3750 switches. can anybody guide with optimal configuration for the 3750 switches for QOS. I would be using Avaya IP phones with PC on the switch ports. Any help highly appreciated
arvindai (TechnicalUser) (OP)
5 May 08 16:48
can anybody suggest good references to get the desired QOS settings on the switch and also means to verify that the QOS has actually been applied. Thanks in Advance
DallasBPF (TechnicalUser)
5 May 08 17:16
Unfortunetly... I am not sure about QoS with Avaya phones, but if you were using Cisco I would just say use the AutoQoS command on the switch ports.  Might give it a try and see how it works;

auto qos voip cisco-phone


Dallas, Texas
CCNA, Net+
Working on MCSA, Convergence+, CCVP

ADB100 (TechnicalUser)
6 May 08 3:45
Best starting point is the QoS SRND found here:
AutoQoS on switches is rubbish IMO, or at least the current macros are, they may change in time but currently I wouldn't touch them.


arvindai (TechnicalUser) (OP)
6 May 08 8:29
Could anybody suggest whats difference between applying mls qos command on the switch and creating class/policy map to mark the qos traffic . thanks
ADB100 (TechnicalUser)
6 May 08 9:14
The policy-map will not work unless QoS is enabled.  When you enable QoS globally it does a few things - it enables the ingress & egress queues and applys some defaults to these queues (i.e. all 4 egress queues get 25% of the scheduler (SRR) and Q1 is additionally shaped to 25% of the links bandwidth).  Enabling QoS also sets all ports as untrusted so any CoS or DSCP markings that are received are reset to 0.


lerdalt (MIS)
6 May 08 9:23
For your phones, you will want to find out if the phone is setting the DSCP value to EF or not.  I'd be surprised if it's not.  If it is, then you will want to put "mls qos trust dscp" on the ports connected to switches.

Otherwise, if you are putting the phones in a separate vlan, you can match based on an acl to set the dscp value yourself.

arvindai (TechnicalUser) (OP)
6 May 08 15:15
I am new onto the QOS things, I have qos settings on my 2950 switches and configuration on the ports is 'mls qos trust cos' . there is no policy map / class map created as such. Now I need to implement on cisco 3750 switches. How should I go to achieve the same
ADB100 (TechnicalUser)
6 May 08 15:31
Simply trusting DSCP on access ports is not advised.  It is VERY easy to discover the voice VLANs and then transmit packets with the Voice VLAN Tag and set DSCP EF from a client PC and therefore hijack the Expedite queues and potentially disrupt your voice traffic.
I would recommend writing ingress QoS policies for access ports and remarking/policing all traffic to fit your QoS policy.  The QoS SRND I mentioned before has example QoS policies (good starting points) for you to analyse and rework for your environment.

arvindai (TechnicalUser) (OP)
6 May 08 20:03
Just to know If I set 'mls qos trust cos' on trunk ports connected to IP phones/PC that configures QOS or not. Is there any way to determine if the packets from the Phones are tagged with QOS or not . Thanks
arvindai (TechnicalUser) (OP)
13 May 08 21:11
could anbody clarify if I don't create policy map to match IP VOIP traffic and simply enables mls qos on the switch, does it provide the QOS marking on the packets for VOIP  
ADB100 (TechnicalUser)
14 May 08 4:33
If you just enable QoS on the switch and do nothing else then any QoS markings that are set that enter the switch will be lost (Layer-2 CoS or Layer-3 DSCP).  This is because the default behaviour of a port when QoS is enabled is 'untrusted', therefore all markings are reset to 0 as they enter the switch.  If you enable 'trust cos' on a port then the 802.1p CoS bits are read and trusted so the packet will recieve priority internally and on egress based on the CoS value.  However CoS can only be set by devices that also write an 802.1q VLAN Tag header, so only trunks or Voice VLAN ports.  If you 'trust dscp' then the same happens, however the switch reads the DSCP value and uses that for internal & egress packet prioritisation.  The DSCP header is found in the ToS field in the IP header so only IP packets will have it.

You need to understand the default DSCP-to-queue and CoS-to-queue mappings to understand how the packet will be treated on egress (show mls qos maps xxxxx).  You also need to understand how each of the (egress) queues are serviced (show mls qos interface queueing).

In my opinion the defaults do not meet the requirements of most VoIP networks.  I would read the QoS SRND on CCO:
This is designed around Cisco Voice, however modifying the policies for any other vendors VoIP equipment is pretty easy.


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