QOS question

[uzumaki]

Hi Guys,

Im sure u are familiar on the bellow command:

Switch(config-if)#mls qos trust cos

I would like to get your opion on the "trust" statement on the command.

The scenario is this I have a 3550 switch, a cisco or avaya ip phone and a pc. The PC is connected to the IP phone and the phone is connected to the switch.

The Question:

When you put the command "trust" does this mean that any incoming packet that has been marked with COS values are being trusted and saves processing cycles in opening the header?

 

[ADB100]

Yes the incoming CoS 802.1p value in the 802.q header will be trusted. This will only be coming from the IP Phone unless the PC is also tagging it's packets. There is no CPU advantage since this is all done in hardware.
Personally I would not configure trust (either CoS, IP Precedence or DSCP) since this is open to abuse from the end station (PC attached to IP phone). This is the easiest approach though.
Have a read of the QoS SRND on CCO for examples of how to configure QoS policies for access switches:

http://www.cisco.com/go/srnd

HTH

Andy

 

[uzumaki]

Hmmm i'll keep that in mind. However for example on the same scenario above, i'll try to use Avaya IP Phones (which by default, it marks its packet with DSCP value of 46 and COS value 6.

I would like to use the dscp as the markings that i trust, what best practice in queing this kind of packets? How will i do my queueng strategy? Wat queueing strategy should fit? Would u mind sharing ur idea?

Thanks.

 

[ADB100]

Short answer is it depends....

Have a read of the QoS SRND at the link I posted. There is also a CiscoPress book 'End-to-End QoS Network Design' by Tim Szigeti which is based on the same material.

Andy