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Traffic Prioritization Involving VLANs

I have a situation that I'd like to run past folks who might have a better understanding of Cisco equipment than I do.

I have a device which will be sending voice and data packets and is able to mark the packets with DSCP values.   Voice, 18 and Data 42.

If this was a straight through network, I'd be clear on how to handle this, but.....

I will be putting this traffic into a VLAN to isolate it from some other traffic on the network. 

What is the best way to prioritize this traffic inside the VLAN?  Will the Cisco switch look at and respect the DSCP values inside the VLAN and prioritize accordingly inside the VLAN?  Or, do I need to do some sort of DSCP to 802.1p mapping?

Another option I would be fine with would be mapping the DSCP values for voice and data to two different VLANs and then giving the voice VLAN priority over the data VLAN .... but I have no idea how to do that.

I'm using 3750E switches.


Frequent Contributor


first of all

do you have some voice quality problems ?

if so then you need to find the causes


No, there are no voice quality issues yet.  I'm trying to avoid having any.  I have 10 meg circuits running between office locations where we send voice and data.  I want to make sure the voice is prioritized so I DON'T have issues.


I'm thinking my best option may be to stuff the traffic into the VLAN and then remap the DSCP value to an 802.1p priority value.   Would that be done with the Cisco class-map command?


Every Interface has an Input and Output Queue, if the packet comes in with a marking and your config says put those packets in a specific queue then irespective of the VLAN,  the packets will be placed into that queue.

You can use port based or MQC based config config, below is a document which explains them in detail.

"I have a device which will be sending voice and data packets and is able  to mark the packets with DSCP values.   Voice, 18 and Data 42."

According Cisco Best practices Voice packets should be marked with DSCP46 (EF- ensured forwarding) and voice signaling - 26 (AF31)


I wasn't aware you could set two DSCP tags on a packet... is that what Cisco is suggesting doing?


Nope, you can only add one tage per packet,  actual voice packets should be marked with 46 and voice signaling packets with AF31. This should be done on your IP phone.

If your phone can't mark the packets and if you use a class-map to match the voice subnet then you can't differentiate between voice and signaling in that case you can mark the entire Voice IP range with DSCP 46


Sorry, I completely mis-read your message.  Yes, OK two different types of packets marked as such.

The phone is doing the DSCP markings.  I'm reading the link you gave now.

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