Specifications

White Paper

Additional defaults can be found in the QoS configuration guides on CCO for a given software release.

7.2 Trusted and Un-trusted Ports

Any given port on the Catalyst 6500 can be configured as trusted or un-trusted. The trust state of the port dictates

how it marks, classifies and schedules the frame as it transits the switch. By default, all ports are in the un-trusted

state. This means that any packet entering that port will have its ToS and CoS rewritten with a zero value.

Consideration must thus be given when QoS is enabled for devices that tag their packets with a priority. Devices

such as IP phones, Call managers, key servers, etc should have their trust setting reviewed.

7.2.1. Un-trusted Ports (Default setting for ports)

Should a port be configured as an un-trusted port, a frame upon initially entering the port will have its CoS and ToS

value reset by the Forwarding Engine to zero. This means the frame will be given the lowest priority service on its

path through the switch. Alternatively, the administrator can reset the CoS value of any Ethernet frame that enters an

un-trusted port to a pre-determined value. Configuring this will be discussed in a later section.

Setting the port as un-trusted will instruct the switch to NOT perform any congestion avoidance. Congestion

avoidance is the method used to drop frames based on their CoS values once they exceed thresholds defined for

that queue. All frames entering this port will equally be eligible to be dropped once the buffers reach 100%. This

process is the same when trust-ipprec or trust-dscp on ingress is used

Note: For Cisco IOS, the software, setting trust is not supported on 1Q4T ports except Gigabit Ethernet ports.

Cat6500(config)# interface gigabitethernet 1/1

Cat6500(config-if)# no mls qos trust

In the example above, you enter the interface configuration and then apply the no form of the command to set the

port as un-trusted.

7.2.2. Trusted Ports

Sometimes Ethernet frames entering a switch will have either a CoS or ToS settings that the administrator wants the

switch to maintain as the frame transits the switch. For this traffic, the administrator can set the trust state of a port

where that traffic comes into the switch as trusted.

As mentioned earlier, the switch uses a DSCP value internally to assign a predetermined level of service to that

frame. As a frame enters a trusted port, the administrator can configure the port to look at either the existing CoS, IP

Precedence or DSCP value to set the internal DSCP value. Alternatively, the administrator can set a predefined

DSCP to every packet that enters the port. This is achieved by attaching an ACL with a predefined DSCP.

Setting the trust state of a port to trusted can be achieved using the following command

With Cisco IOS the setting of trust can be performed on a Gigabit Ethernet interface, 10GE interface, 10/100 ports

on the WS-X6548-RJ45/21 line card and 100Mb ports on the WS-X6524 100FX ports.as well as the 6148/6548-GE-

TX linecards)

Cat6500(config)# interface gigabitethernet 5/4

Cat6500(config-if)# mls qos trust ip-precedence

This example sets the trust state of Gigabit Ethernet port 5/4 to trusted. The frames IP precedence value will be used

to derive the DSCP value.