QOS Confusion

[keithja]

There is a long-standing question in my mind in regards to QOS on Cisco switches (specifically in my case, 3560s and 3750s) that I hope someone can clear up for me once and for all.

In the document:

http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_m2.html#wp1041723

I see the following statement:
QUOTE
All of the CoS-to-DSCP and DSCP-to-CoS maps are globally defined. You apply all maps to all ports.

If you enter the mls qos trust cos command, the default CoS-to-DSCP map is applied.

If you enter the mls qos trust dscp command, the default DSCP-to-CoS map is applied.

End QUOTE

Yet I am forever-no-end seeing the following configuration for interfaces:
"
mls qos map cos-dscp 0 8 16 40 32 46 48 56
mls qos trust dscp
"

including in the "Nortel IP Phone Inter-Working with Cisco L2 Switches" document.

I am clearly not understanding something. It seems to me that if 'mls qos trust dscp' is set, 'mls qos map cos-dscp ' anything, becomes null and void, and the only map that matters is the 'dscp to cos' map.

Can someone clear this up for me? It is a point of confusion every time I hit it.

Thanks!

 

[unclerico]

remember the maps are global where the trust state is specified per interface. so on one interface you could have mls qos trust dscp and then on another you could have mls qos trust cos. altering the default cos-dscp map is necessary so that if you do trust the cos values on an interface the proper dscp value is derived from the map.

 

[keithja]

Hi UncleRico,

That would make sense, but the cases in which I've seen it (the Nortel document is a perfect case in point) it is clearly talking about the settings in reference to the same port. To the effect of:

"
In global configuration enter the command "mls qos map cos-dscp 0 8 16 40 32 46 48 56"

Then for each port to which an IP phone is connected enter the following configuration:
voice vlan 10
mls qos trust dscp
priority-queue out

For the uplink port enter etc, etc.

"

So am I thinking about something incorrectly, or did they make a mistake? When viewing it as configuration setup for an IP phone on a cisco switch - which is what it seems they are referring to - wouldnt it make more sense, when using trust dscp - to use 'mls qos map dscp-cos (and the required remappings)?

thx

 

[liliahrj]

Hi, How i can to apply QoS in switches Cisco 3560,3750 and 4000: I need to do the next in these switches:

policy-map QoS-policy
description Aplicacion de prioridad

class Voice-Services-DSCP
priority 32
compress header ip rtp

class Mission-Critical-DSCP
bandwidth remaining percent 50
random-detect dscp-based
random-detect ecn
random-detect dscp 24 34 40

class Interactive-DSCP
compress header ip tcp
bandwidth remaining percent 7
random-detect dscp-based
random-detect ecn

class No-Estandar-QoS
bandwidth remaining percent 8
random-detect dscp-based
random-detect ecn

class class-default
bandwidth remaining percent 35
random-detect dscp-based
random-detect ecn

Please somebody can help me
Thanks

 

[ADB100]

Not possible with the 3560/3750 - not too sure with the Cat 4000? The policy you show looks like it is an egress policy from a 'real' router. With the 3560/3750 you can only apply QoS policies inbound and your options are to classify (or re-classify) the traffic to a DSCP or CoS value, police to a rate and then either mark down (i.e. DSCP 0 -> 8 when the rate is exceeded) or simply drop. You cannot apply egress policies but you can queue traffic into any of the 4 queues based on DSCP or CoS value (dscp and cos to queue maps).

Andy