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QoS setup on SGE2010 to support VoIP trunks between PBXs

Answered Question
Aug 29th, 2012
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Folks, I'm stumped on how to configure QoS on my SGE2010 to support my Toshiba CIX PBX VoIP trunks.  The infrastructure is straight forward:

PBX(1)<==>SGE2010(1)<==>SGE2010(2)<==>SGE2010(3)<==>PBX(2)

where PBX(1) and PBX(2) are connected through three SGE2010's that are trunked via fiber between buildings.  I don't have VoIP handsets and the PBX's are on their own VLAN.  Everything else has been a breeze in these switches but I have no experience with QoS and the admin guide doesn't help.

Jim

Correct Answer by Tom Watts about 4 years 11 months ago

Hi Jim,


The Basic QoS feature should be all you need for the switch. The QoS works for ingress traffic only. Basic QoS behavior dictates, if you have upstream QoS assigned, the switch will trust the values as is. If there is no QoS upstream, the ingress traffic will be marked based on how the traffic is classified. Any traffic egress of the switchport will not be remarked.


On a flat network, 1 vlan, the default CoS value 0 which is relevant for untagged packets. You may modify these values 1-7 for untagged traffic. Reference the CoS tables to know the values 1 through 7 mean.


The strict priority for the QoS queue will depend on how the traffic is marked. With strict priority, the highest value traffic will be allocated first on the egress port. So you should leave this alone.


The part that most have problems with, the DSCP values. These are defined on the DSCP to queue. You will notice a queue next to a DSCP mark. This traffic is classified on what the packet contains. Voice traffic is generally DSCP 46, which by default is marked with a higher queue, I believe 3 by default. The switch will affix the DSCP value for the ingress traffic or trust the ingress mark. But it will not remark egress traffic.


I hope this somewhat clarifies your inquiry.



-Tom
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Correct Answer
Tom Watts Wed, 08/29/2012 - 12:46
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Hi Jim,


The Basic QoS feature should be all you need for the switch. The QoS works for ingress traffic only. Basic QoS behavior dictates, if you have upstream QoS assigned, the switch will trust the values as is. If there is no QoS upstream, the ingress traffic will be marked based on how the traffic is classified. Any traffic egress of the switchport will not be remarked.


On a flat network, 1 vlan, the default CoS value 0 which is relevant for untagged packets. You may modify these values 1-7 for untagged traffic. Reference the CoS tables to know the values 1 through 7 mean.


The strict priority for the QoS queue will depend on how the traffic is marked. With strict priority, the highest value traffic will be allocated first on the egress port. So you should leave this alone.


The part that most have problems with, the DSCP values. These are defined on the DSCP to queue. You will notice a queue next to a DSCP mark. This traffic is classified on what the packet contains. Voice traffic is generally DSCP 46, which by default is marked with a higher queue, I believe 3 by default. The switch will affix the DSCP value for the ingress traffic or trust the ingress mark. But it will not remark egress traffic.


I hope this somewhat clarifies your inquiry.



-Tom
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jroot Thu, 08/30/2012 - 10:42
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Tom,


I found this Cisco doc http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/19ew/configuration/guide/qos.pdf which helped me understand what you were saying and yes, that did clarify things.  So here is what I did:

1) I reverted my config back to before I started messing around with QoS settings.

2) I verified QoS was in Basic mode.

3) I set the ports on each switch to CoS 5 which equates to DSCP 46.

 

4) Didn't touch Queue and left Strict Priority in place.

5) CoS to Queue defaulted CoS 5 to Queue 3.

6) DSCP 46 is in fact assigned to queue 3 by default in the switch, as you stated, so no mismatches.

7) No Bandwidth or VLAN Rate Limits were set.


The result of these settings should be ingress traffic from ports with CoS of 5 to receive strict priority and allocated first for egress.  As long as my QoS settings are identical for all switches between the PBX's, my VoIP traffic should have priority across the switch trunks.  Am I missing anything?

Tom Watts Thu, 08/30/2012 - 11:05
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If you're marking the traffic ingress on the switchport, your other devices simply need to trust the mark. In other words, no configuration aside "assuring trust mode".



-Tom
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jroot Thu, 08/30/2012 - 11:24
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From the lcli on the switch I did a "show qos":


console# show qos

Qos: basic

Basic trust: vpt

console#


And in the admin gui:


That's what I should see on each switch for "assuring trust mode" correct?

Tom Watts Thu, 08/30/2012 - 11:29
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Yes, it can either be the CoS or DSCP.  CoS is the 1-7 value, the DSCP is the mark value.


-Tom
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jroot Thu, 08/30/2012 - 12:34
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Tom, I will give this a shot and let you know how it works when I get these into production.  Thanks a heap!

Jim

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