QOS question on switches when using mls qos trust dscp

[carl_townshend]

Hi all

some questions about qos

first of all, I have seen that my default the priority queue is not enabled on switch ports unless i configure it, is this correct?

Will voice traffic use the priority queue?, how do I know this?

also if I use the mls qos trust dscp command, this should read the tags from the voice traffic and put i the appropriate queue? if I dont enable the priority queue will this command effectively have no use ?

cheers

[jonathanaxford]

Hi Carl,

The "mls qos trust dscp" command tells the switch to trust any incoming DSCP values that it receives in the frames. If an IP telephone handset is marking its voice traffic as DSCP EF, then the switch will use this value in its QoS scheduling.

You are essentially saying that you trust any incoming packets to be marked correctly.

It depends on how you have QoS setup on the switch, but using something like Auto Qos as an example, then if a priority queue is enabled and voice traffic is marked as DSCP EF, then it will be placed in the priority queue.

So even if you don't have the priorty queue enabled, the mls qos trust dscp command may still be needed to ensure that DSCP markings are left intact across the network.

What QoS configuration/device are you using?

Many thanks

Jonathan

[carl_townshend]

Hi There

basically, we have the mls qos global command enabled, and then on the voip ports and switch uplinks to the core we use the mls qos trust dscp command.

will this be working as desired, ie putting the voip traffic in the priority queues ?

cheers

[jonathanaxford]

Hi Carl,

It doesn't sound like there will actually be any QoS prioritisation going on, all that is happening is that received DSCP markings are being trusted by your switch. What model of switch are you using?

Many thanks

Jonathan

[carl_townshend]

Hi, it is the cat 2960 model.

So what would I need to do to fix this ?

[Shashank Singh]

Hi Carl,

The best option is to enable 'auto qos voip trust' on the interfaces connecting the ip phones. This is a cisco feature that utilizes QoS to prioritize voice traffic.

Other options are to use 'mls qos trust dscp' on the interface and this should take care of the prioritization in case the traffic is coming pre-marked  with EF dscp.

You can verify if QoS is working for you by checking 'sh mls qos interface fax/y stat' output. Make sure the counters for the correct dscp (generally 46 for voice) is incrementing while you receive voice traffic.

'Priority queue out' can be done under any interface on this platform and this will take all the traffic coming out of that interface to the priority queue, irrespective of the marking.

Hope this helps,

Shashank

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[carl_townshend]

Hi there

what about the uplinks to my core? surely I would want to enable the priority queue out on these? or would you do the auto qos voip trust command on all ports including uplinks to core ?

[Shashank Singh]

Hi Carl,

auto qos voip trust is supposed to be configured only on access ports which connect IP phones. On the uplinks, you should configure 'mls qos trust dscp' to trust the dscp marking and prioritize accordingly.

If you enable 'priority queue out' on the uplink, that will take all the egress traffic on the uplink (irrespective of the marking) to the priority queue.

Cheers,

Shashank

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[jonathanaxford]

Hi There,

Are you using Cisco IP phones? If so, you can use the "auto qos voip cisco-phone" command on each access port that connects a telephone, followed by the mls qos trust dscp if you wish to trust dscp markings (By default, auto qos will set the port to trust cos markings).This command uses CDP to ensure that the attahed device is actually a telephone before trusting the received QoS markings.

If you are not using Cisco phones, you can simply use the auto qos voip trust command on the access ports, followed by mls qos trust dscp.

Then, on each uplink to another trusted switch/router, use the "auto qos voip trust" command. This will set up all of the necessary QoS paramters and ensure that voice traffic is correctly placed into the priority queue.

I have used this model for a while and it tends to work very well for most deployments, it also keeps it simple!

Hope this helps

Many thanks

Jonathan

[carl_townshend]

Hi there

So are you saying if I just have the mls qos trust dscp set on my switch, will this just trust the frames/packets that come in, but it will not do anything special to schedule them?? cause if I dont enable the priority queue my important traffic will just be serviced the same as the other traffic?

so really i need to enable the priority queue for my voice ?

cheers

[jonathanaxford · Accepted solution]

Hi Carl,

That's exactly right - the mls qos trust dscp command on its own does not setup any prioritisation of the incoming packets - it merely tells the switch to trust the values that it finds in the IP headers.

I would recommend the auto qos config for a 2960 platform, that should make sure that all of your voice traffic is prioritised correctly,

Many thanks

Jonathan

[carl_townshend]

Hi there

I have done this, However the 2960's connect to my core cat 6509 chassis, what QOS config should I be putting on the core?? would the mls qos trust dscp be enough, or would I also need to enable the priority queue on each of the interfaces that connect to the edge switches?

cheers

Carl

[Shashank Singh]

Hi Carl,

On the interfaces on 2960 switches which directly connect the phones, configure auto qos voip trust. On the uplinks on the core switches, just configure mls qos trust dscp.

Cheers,

Shashank

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[carl_townshend]

Hi there

but on the uplinks, this means the priority queue wont be enabled, do i need to do this?

also what about my 6500 core switches?

cheers