Qos on 3750

Answered Question
Dec 3rd, 2009

Hi All,


             I have been reading the qos on 3750 docs. I am currently planning to implement voip on the infrastucture. The IP handsets are Nortel.

My question is in reagrds to the priority-queue out command.


Now when this command is configured on an inteface that is connected to the handset, it basically enables the egress priority queuing. But is this egress queuing is enabled only for that port. Now if I have the same command on the uplinks, then I assume this is what should happen :


* both the input port ( where the phone is connected ) and the uplink port are configured with priority-queue out command. Omit the rest of config for time being.


* packet comes in gets classifed say cos 5 so get priority input queue. so gets the processing first by the SRR.


* this packset/frame will be analsyed by switching fabric as to where to go based on the output map table.


* now the frame is sent to the correct port for egress, queuing will see  the frame cos 5 again( no mutation) and put it in the priority output queue.


* since the priority egress the this transmit queue will be sent out first.


The above will happen only if the uplink also has the priority command in it, if it doesnot then all the queue will be in the shared mode. Hope my understanding is correct.


Sorry if it sounds a bit silly .  I am just confirming my understanding as I am qos newbie.


Thanks.

Correct Answer by Edison Ortiz about 7 years 2 months ago

prakadeesh wrote:


Thanks jon .


So expedite queung is recommended on the uplinks and the ports connecting to the phones dont need this commands( what i understand). But in the case of an incoming call to this port dont we still need the expedite queueing enabled on the access ports, so that we dnt have any delay.


Thanks.

Yes, you need this command on user facing ports. You will most likely have a PC attached to the phone and if the PC is downloading a huge file, you want to make sure the voice packet gets priority on egress and delay the PC download as needed.


If you have QoS enabled in a switch, it is recommended to enable expedite queueing in all switchports.


Regards


Edison.

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Jon Marshall Thu, 12/03/2009 - 05:15

prakadeesh wrote:


Hi All,


             I have been reading the qos on 3750 docs. I am currently planning to implement voip on the infrastucture. The IP handsets are Nortel.

My question is in reagrds to the priority-queue out command.


Now when this command is configured on an inteface that is connected to the handset, it basically enables the egress priority queuing. But is this egress queuing is enabled only for that port. Now if I have the same command on the uplinks, then I assume this is what should happen :


* both the input port ( where the phone is connected ) and the uplink port are configured with priority-queue out command. Omit the rest of config for time being.


* packet comes in gets classifed say cos 5 so get priority input queue. so gets the processing first by the SRR.


* this packset/frame will be analsyed by switching fabric as to where to go based on the output map table.


* now the frame is sent to the correct port for egress, queuing will see  the frame cos 5 again( no mutation) and put it in the priority output queue.


* since the priority egress the this transmit queue will be sent out first.


The above will happen only if the uplink also has the priority command in it, if it doesnot then all the queue will be in the shared mode. Hope my understanding is correct.


Sorry if it sounds a bit silly .  I am just confirming my understanding as I am qos newbie.


Thanks.


Prakadeesh


When the packet comes into the port attached to the phone the "priority-queue out" configured on that port has no effect because of the direction of the packet. You can configure a priority queue on the ingress by using the command "mls qos srr-queue input priority-queue .." but note that this is a global command and not an interface specific command.


So when the packet comes in with a value of CoS5 the "priority-queue out" configured on that interface has no effect. If the packet was going out of that port it would have an effect.


Assuming all your maps are setup correctly the packet will then go to the uplink port and will be placed in the priority queue  because of the configured "priority-queue out" interface command on the uplink port.


It's important to bear in mind the direction of packets when dealing with QOS and especially with Catalyst switches because the ingress and egress queues often have different capabilities.


Jon

prakadeesh Thu, 12/03/2009 - 05:22

Thanks jon .


So expedite queung is recommended on the uplinks and the ports connecting to the phones dont need this commands( what i understand). But in the case of an incoming call to this port dont we still need the expedite queueing enabled on the access ports, so that we dnt have any delay.


Thanks.

Correct Answer
Edison Ortiz Thu, 12/03/2009 - 13:08

prakadeesh wrote:


Thanks jon .


So expedite queung is recommended on the uplinks and the ports connecting to the phones dont need this commands( what i understand). But in the case of an incoming call to this port dont we still need the expedite queueing enabled on the access ports, so that we dnt have any delay.


Thanks.

Yes, you need this command on user facing ports. You will most likely have a PC attached to the phone and if the PC is downloading a huge file, you want to make sure the voice packet gets priority on egress and delay the PC download as needed.


If you have QoS enabled in a switch, it is recommended to enable expedite queueing in all switchports.


Regards


Edison.

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