default QoS behavior on 4500- IOS

Unanswered Question
Dec 9th, 2008

By default QoS is disabled. But what is its exact behaviour?

I've read that DSCP is trusted, COS untrusted when QoS is disabled.

What will be the result for the queuing on the egress queue?

Will packets scheduled based on the Cos value? this means:


COS to DSCP mapping=> DSCP 0

DSCP to Tx queue=> Tx 1 for all traffic and DSCP value set to 0

or the DSCP value keeps maintained and the default DSCP to Tx queue mapping is used?

I've no hardware to test and I don't find the exact information online

I have this problem too.
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Collin Clark Tue, 12/09/2008 - 07:06

Per the QoS SRND, With QoS disabled, the port interface trust states default to trust DSCP. With QoS enabled, the trust state changes to untrusted and with no other QoS parameters explicitly configured, all egress traffic is marked with CoS 0 and DSCP 0. The queuing when QoS is off will be the queuing mechanism you have configured on the switch. I believe the default is WFQ.

Hope that helps.

davy.timmermans Tue, 12/09/2008 - 07:13

Thus with QoS disabled,

the 4 queues are used and scheduled according to the DSCP of the IP packet (unchanged)


1)if I attach a pc to a switch with Qos disabled

2)ping to another switch also with QoS disabled

3) The DSCP value will be unchanged

or will it be marked down to 0

Now material to test at the moment :(

davy.timmermans Tue, 12/09/2008 - 07:35

I was confused by the following text


QoS is globally disabled (default), the incoming packet's DSCP value is trusted. However a Layer 2 packet's class-of-service (CoS) value would be

considered untrusted and a CoS value of zero would be used. This CoS value of 0 also maps to a DSCP value of zero.

probably an error


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