Some QoS Queries 1

[cyberspace]

Just got a few questions regarding Quality of Service over Cisco equipment.

I work with the 3Com VCX IP Telephony Suite and part of our setup involves traffic prioritization for voice traffic over WAN links (if it's a multi site organization).

Voice traffic runs over it's own VLAN, and the phone ports are tagged in this vlan.

With a 3Com 5500G switch, there are 8 buffers per port, one of which is EF (Expidited Forwarding) which has a DSCP value of 46 (standard for voice I believe) and is the highest priority traffic. The phones tag the packets with this value (sorry if i'm stating the obvious here)

It is my understanding however that Cisco does not recognize the DSCP value of 46, so if a tagged packet arrives at an interface with this value, it will not honour it and so loses it's prioritization (please correct me if I am mistaken, I would like to know exactly, rather than vaguely!)

Having read about QoS/DSCP on Cisco (

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml)

my query is - how do we get Cisco equipment to uphold the values of tagged packets? Am I correct that we can improve efficiency by not checking packets from the voice VLAN and trusting the data so that it is not modified? But then, how would the Cisco router know what to do with a packet with a DSCP value of 46?

I am fairly new to Voice QoS, so more detailed information, corrections etc are most welcome.

Thanks

'When all else fails.......read the manual'

 

[BuckWeet]

I assume you are talking about a switch..

There are two ways.. disabled QoS globally on the switch (doubt you want this) or on the port level you can do 'mls qos trust dscp' or it might also be 'qos trust dscp'

It depends on the switch model..

 

[BuckWeet]

Routers don't rewrite DSCP/TOS information by default. You have to configure them to do this. However, switches do.


Sorry forgot to add that

 

[cyberspace]

Thanks for that. Yes, they are switches. The particular model being used for voice is a 2950 PoE I believe.

I was also told that we would need to inform the customer to request their MPLS provider to also allow this traffic otherwise again, it would lose it's value. Is this the case?

The global option isn't what I'm after but the port level certainly sounds like it is - so thanks for that.

If there is anything else anybody can add, please do so

'When all else fails.......read the manual'

 

[cyberspace]

oh also i'm particularly interested in finding out how the switches know what to do with a packet with a DSCP value of 46 - how does it know it's the highest priorty when it doesn't use the values itself (ie with Cisco)


'When all else fails.......read the manual'

 

[ADB100]

You really need to find the specific model number of the switch as there is no real consistency of QoS configuration between different Catalyst plaforms. You mention a 2950 PoE, this is obviously wrong as there isn't a PoE 2950.

Andy

 

[BuckWeet]

There is also a command 'no mls qos rewrite' i believe that is a global option you might want to look into. If you don't issue this command it will could rewrite your tags based on the CoS map..

To answer your question about DSCP46, since its a layer2 device, but layer3 aware; it will read this tag and use its DSCP to CoS map to process it.. DSCP46 will be mapped to CoS 5 I believe (don't quote me on that) and forward it based on the L2 CoS value.. This is where the above 'qos rewrite' mapping comes into play.


BuckWeet

 

[ADB100]

Buckweet, depending on the 2950 model it might not be layer-3 aware. The 2950 has a single IOS image but different features are enabled depending on the hardware platform. If it's an SI only capable switch then you don't get any layer-3 awareness (with QoS or security ACLs), if its an EI capable switch then you have some layer-3 awareness, however not all DSCP values are recognised.

It may not be a 2950, however since he stated it was PoE?

 

[cyberspace]

Sorry guys, I was mistaken about the 2950 being PoE, there are 2950's in place however:

WS-C2950-24
WS-C2950C-24
WS-2950SX-24

The PoE switch is WS-C3560-24PS

The majority of the phones are running from the 3650 but there are Voice VLAN ports on the listed 2950's

Thanks for the help so far, most useful.

'When all else fails.......read the manual'

 

[cyberspace]

Just thought i'd bump this, as I could still use some help! thanks guys, info so far has been very helpful

'When all else fails.......read the manual'