4500 QoS default behaviour

Unanswered Question
Sep 12th, 2008

I have a client who has forwarded there config for a 4500 router and asked wether QoS is actually doing anything with the applied configuration, relevant parts shown below:-


qos map cos 4 to dscp 34

qos map cos 5 to dscp 46

class-map match-all Trusted_Device-1-class

match access-group name Trusted_Device-1



policy-map Trusted_Device-policy

class Trusted_Device-1-class

trust dscp

interface GigabitEthernet7/4

switchport trunk encapsulation dot1q

switchport mode trunk

load-interval 60

qos trust cos

service-policy input Trusted_Device-policy

As far as I can see the policy is only saying "TRUST DSCP MARKINGS" but nothing else.

With QoS enabled, what is the default queuing method, and would the above be enough to prioritise VOIP traffic using IP5 or DSCP46.

Kind Regards


I have this problem too.
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Giuseppe Larosa Fri, 09/12/2008 - 07:35

Hello Tony,

in order to correctly support VoIP traffic you would need a scheduler using LLQ.

to be noticed that the qos trust cos should already be doing the job of the service-policy:

in input it trusts the L2 CoS values

internally the L2 CoS are mapped to DSCP values

the service-policy does this by using input DSCP and use it to create the internal DSCP.

in the outgoing interface queueing should be done based on the internal DSCP tag after it is mapped again to a CoS.

Strict Priority / Low Latency Queueing

You can configure transmit queue 3 on each port with higher priority using the priority high tx-queue configuration command in the interface configuration mode. When transmit queue 3 is configured with higher priority, packets in transmit queue 3 are scheduled ahead of packets in other queues.

When transmit queue 3 is configured at a higher priority, the packets are scheduled for transmission before the other transmit queues only if it has not met the allocated bandwidth sharing configuration. Any traffic that exceeds the configured shape rate is queued and transmitted at the configured rate. If the burst of traffic, exceeds the size of the queue, packets are dropped to maintain transmission at the configured shape rate.

use this document as a reference


I think you need to modify queueing in order to correctly handle VoIP using LLQ.

see the section "configuring transmit queues"

Hope to help


tonymurphy30 Fri, 09/12/2008 - 15:51

Many thanks for your information.

I have looked through at the default queuing algorithm for the switch without using the "prioity high" command in it is only Round Robin, not even WRR !

I will advise that they change queue 3 to a high priority queue on the relevant interfaces.

It's delicate ground as we support the voice (Avaya CM4) and a very large ISP supports their data infrastructure, but we have been asked to comment on the existing set up.




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