mls qos trust{cos/ip-precedence/dscp} command

Answered Question
Jan 1st, 2009

Hi every body!

I have few questions

1)

The command " mls qos trust dscp" is only valid on mulilayer switch or it is also valid for layer 2 switch? If layer 2 switch is configured with that command, can it modify the dcsp value based on policy?

2)is the following correct:

switch(config-if) mls qos trust dscp

switch will set the cos value to set default. If the default set is zero, then frame will be processed by best-effort delivery.

But the egress-queue will be decided by dscp value in the packet. A dscp to cos map will be used to drive the cos value and then frame will be placed in the queue that corresponds to cos value.( off course if egress port is configured for trunk)

thanks a lot and I wish America and all of you a happy new year!

thanks a lot!

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Correct Answer by Jon Marshall about 8 years 3 weeks ago

Sarah

1) L2 switches can trust the dscp marking as well. The 2960 is a layer 2 only switch and the default is untrusted but if you then enter

"mls qos trusted" you have a choice of 'cos|dscp|ip-precedence'. The default if no choice is entered is DSCP.

2) If "mls qos trust dscp" is entered then the switch will use the DSCP marking found in the packet. This will then be used as the internal DSCP marking that all switches use. Unless you have a DSCP-DSCP mutation map the value used will be the value received in the packet.

Jon

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Correct Answer
Jon Marshall Fri, 01/02/2009 - 14:14

Sarah

1) L2 switches can trust the dscp marking as well. The 2960 is a layer 2 only switch and the default is untrusted but if you then enter

"mls qos trusted" you have a choice of 'cos|dscp|ip-precedence'. The default if no choice is entered is DSCP.

2) If "mls qos trust dscp" is entered then the switch will use the DSCP marking found in the packet. This will then be used as the internal DSCP marking that all switches use. Unless you have a DSCP-DSCP mutation map the value used will be the value received in the packet.

Jon

sarahr202 Sat, 01/03/2009 - 10:57

Thanks for your reply and my apology for posting question in wrong forum.

The question is When switch receives frame but switch is configured to trust dscp, would the switch use the default cos=0 to place the frame in the respective queue at ingress port?

or would switch drive cos value from dscp-cos map and place the frame in respective queue at ingress port?

At egress port, would switch configured to trust dscp, again use the dscp value to drive cos from dscp-cos map to place frame in respective queue ?

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