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QoS on 6000

A very quick question about QOS.

After studying a number of pdf files, Im still unsure of how the PFC derives the DSCP value for a frame.

In this scenario a trunk port has been configured as a trusted port, and its trusting the cos value contained within the frame.

Set port qos x/y trust trust-cos

The trunk port has also been configured for vlan-based .

Set port qos x/y vlan-based

There is a number of vlans on these trunk ports.

All the documents that I’ve read indicate that the Switching Engine (PFC) derives its internal DSCP value from one of four options.

1. An existing DSCP value set prior to the frame entering the switch

2. The received IP precedence bits already set in the IPV4 header. As there are 64 DSCP values and only eight IP precedence values, the administrator will configure a mapping that is used by the switch to derive the DSCP. Default mappings are in place should the administrator not configure the maps.

3. The received CoS bits already set prior to the frame entering the switch. Similar to IP

precedence, there are a maximum of eight CoS values, each of which must be mapped to one of 64 DSCP values. This map can be configured or the switch can use the default map in place.

4. Set for the frame by using a DSCP default value typically assigned though an Access Control List (ACL) entry.


If the Trunk port is configured for trust-cos, therefore a trusted port, will the PFC derive its DSCP value from the COS or will it use the default acl because no acl’s have been assigned to any Vlan’s on that trunk.

There are known limitations with regards to older line cards (10/100) that do not support the command set port qos x/y trust trust-cos, therefore an ACL needs to specified in order to correctly classify ingress frames.

However, no mention is made with regards to whether ACLs are required for newer line modules (copper or fibre) when the command is supported. Should the DSCP value classified on ingress be maintained through to the PFC when an ACL is not attached to the same Vlan ? Is the frame still classified by the default ACL ?

Cisco Employee

Re: QoS on 6000

Re: QoS on 6000

Reclassification is not required on the ingress port when the port is configured to trust-cos, therefore the mapped dscp value will be maintained through the PFC, and ignor the default or an attached ACL.

Reclassification will occur when the port is not trusted, and the default ACL will only be used if an ACL is not already attached. The default ACL will reclassify with COS with a value of zero.

Trust this helps.