Auto QOS on 3750

[Phil Bradley]

I have enabled autoqos for voice on my 3750's and have a question regarding operation. I have a layer 3 interface defined on the 3750 using the no switch port command and this connects eventually to another layer 3 port on the other end. I captured traffic in Wireshark and specifically voice traffic on the egressg of this layer 3 interface and noticed the DSCP bit is getting set to expedited forwarding and at layer 2 there is no pbit marking. The device in between the 3750 router interfaces is a fiber switch and it can map the dcsp bits into a queue at layer 2 and handle the voice data appropriately. My question is about the behaviour of auto Qos voice on the Cisco side, and is it normal to only see the dcsp marking and no layer 2 pbit? I assume if I captured on a switchboard on the 3750 then I would see a layer 2 pbit. Is the pbit getting stripped at the layer 3 boundary? Thanks!

[Joseph W. Doherty]

L2 QoS marking is generally done using a VLAN tag field. I.e. no VLAN tag, not L2 QoS marking.

BTW, the 3750 series can utilize L3 ToS even on "ordinary" L2 only interfaces.

Cannot say what your fiber switch can, or cannot do, but would suggest using L3 ToS, if available.

Also BTW, believe a 3750 can use L3 ToS to create L2 CoS, and the converse too.

[Phil Bradley]

Hi Joseph,

Thanks for the info! Yes, the p-bit that I am referring to is in the vlan portion and is defined in the 802.1p standard. I didn't see this info in my capture, but I was capturing on the fiber switch side. 

I know my fiber switch will honor the L3 qos and basically map it to a queue and depending on the marking it will prioritize the frame accordingly.

I guess I need to capture on the 3750 and look at my voice vlan to see what's happening there. Since it's at layer 2 I assume it's using 802.1p unless Cisco is doing something different with their auto Qos.

[Joseph W. Doherty]

802.1p does require a VLAN tag. If you voice VLAN is an edge port voice VLAN, you should see the VLAN tag on its frames, but for non-access ports, they would need to be a trunk port to contain that VLAN tag.

I forget what a 3750's auto QoS does, by default, with 802.1p. Personally, I've normally used just the L3 ToS, as again, no need to worry about it being lost at every L3 hop which builds a new frame, often without a VLAN tag.

VoIP phones will vary, by default, in what they will do for L3 and L2 QoS tagging. Usually the VoIP phone will set both a L3 ToS and L2 CoS.

[Phil Bradley]

Ah, yes that makes sense. Once I hit the router interface then the vlan tag disappears. So it sounds like either the auto qos is applying the DCSP bit and using it from start to finish, or it's adding this at the L3 boundary. I guess I could mirror the L2 interface and check to see if the pbit is in the vlan tag there and the L3 marking as well.

Regardless of L2 or L3 markings, I guess it depends on what the switch or router does with this, correct? I assume just because I see it doesn't mean it will get utilized.

I feel pretty comfortable with the fiber switch actions. It will basically look at the L3 bit, map it to a switch queue, and then process these as high priority. It sounds like from what you are saying above that the 3750 can do this l3 to l2 mapping as well, but what auto Qos voice does is up in the air. I guess I need to look up what auto Qos actually does. Thanks for your help on this!

[Joseph W. Doherty]

". . . I guess it depends on what the switch or router does with this, correct?"

Yes, very much so, Most Cisco devices, by default, don't do any "QoS" unless configured to do so. Or course, when auto QoS is enabled, you're invoking a set of pre-arranged QoS configuration features.

Auto QoS might work very well for you, or it might not. It really depends how suitable it's for your traffic and QoS needs.

"I assume just because I see it doesn't mean it will get utilized."

If I remember correctly, on a 3750 the answer for that is also yes. You can have QoS configuration statements, that are unused if QoS is actually disabled. If so, this is to allow you to disable/enable QoS without needing to have to reenter a whole QoS configuration.