Cisco SWITCH Campus VoIP Refresh. Part 3b – QoS Configuration

I can’t believe I’ve had to chop up not just the VoIP refresh section into three parts, but part three into A and B!

It’s surprised me a little and I must have a little more to say on the subject than I thought when I started putting finger to key last night. But, I AM trying to keep this in scope for the SWITCH exam, so we’ll discuss just the req’s for that in this article.

Configuration

First of all, in order to turn on QoS processing on the switch, we need to enable it with the mls qos global command.
This is something that’s easy to overlook as you’ll enter interface commands all day long, without this, they don’t count for anything.

SW3#sh mls qos
QoS is disabled
QoS ip packet dscp rewrite is enabled
SW3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW3(config)#mls qos
SW3(config)#do sh mls qos
QoS is enabled
QoS ip packet dscp rewrite is enabled

Notice the result of the first command stating that QoS is disabled.
Then the show command entered shows the processing being turned on globally.

Under your switchport interfaces, there are many things that could be done, but as mentioned previously, keeping a tight scope to the SWITCH exam.

To demonstrate a few things here, I’m going to show that the interface is in default configuration and then apply four different commands, two of which are setting the VLAN id’s for Access and Voice VLANs.

SW3(config-if)#do sh run int fa 0/1
Building configuration...
Current configuration : 33 bytes
!
interface FastEthernet0/1
end
SW3(config-if)#switchport host
switchport mode will be set to access
spanning-tree portfast will be enabled
channel group will be disabled
SW3(config-if)#switchport access vlan 11
SW3(config-if)#switchport voice vlan 22 
SW3(config-if)#auto qos voip trust
SW3(config-if)#do sh run int fa 0/1
Building configuration...
Current configuration : 235 bytes
!
interface FastEthernet0/1
 switchport access vlan 11
 switchport mode access
 switchport voice vlan 22
 srr-queue bandwidth share 10 10 60 20
 priority-queue out 
 mls qos trust cos
 auto qos voip trust 
 spanning-tree portfast
end

So hopefully you can see, from four commands, I received eight lines of configuration.
That’s because the cheeky switchport host command is a macro which sets a port up for an end device, simlarly the auto qos voip trust command is the Auto-QoS command for non-Cisco IP-Phones to be attached to your access interfaces.

From an real world operational standpoint, Auto QoS is pretty much all you need for your access ports, the only thing that I’ll mention is, try and ensure you’re running the same release of IOS on all your particular switch models as QoS maps may be different between releases.

That’s the automatic portion of configuration covered, to just expand a little on manual configuration for the exam…

To configure an IOS switch to trust the markings on traffic entering an interface, use the following:

Switch(config-if)# mls qos trust {dscp | cos}

To configure the switch to trust the traffic markings only if a Cisco phone is connected, use the following:

Switch(config-if)# mls qos trust device cisco-phone

To set a COS value for frames coming from a PC attached to the phone, use the following:

Switch(config-if)# switchport priority extend cos <cos-value>

To verify the QoS parameters on an interface, use the following:

Switch(config-if)# show mls qos interface <interface>

Here we use the last command mentioned to see how the show command interprets the Auto-QoS settings of the commands we used earlier and a second interface which is default, for comparison (no devices are attached at this point).

SW3#sh mls qos interface fa 0/1 
FastEthernet0/1
trust state: trust cos
trust mode: trust cos
trust enabled flag: ena
COS override: dis
default COS: 0
DSCP Mutation Map: Default DSCP Mutation Map
Trust device: none
qos mode: port-based
SW3#sh mls qos interface fa 0/2
FastEthernet0/2
trust state: not trusted
trust mode: not trusted
trust enabled flag: ena
COS override: dis
default COS: 0
DSCP Mutation Map: Default DSCP Mutation Map
Trust device: none
qos mode: port-based
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s