HP 1920-24G and DSCP 46

[PiotrIr]

Just wander if somebody could help me to find answer for my problem. I mark IP packets on device connected (laptop’s NIC) to HP 1920-24G with DSCP 46 – so packet which ender switch is marked with Expected Forwarding value. Do I need to do anything else to make sure this packet will be prioritized and send out as soon as possible or it will work automatically? Will packed has the same DSCP value when exit AP or I have to mark it again on the device?

[Vince-Whirlwind]

A few points:

The switch will forward all frames as fast as possible. Setting EF on your packets doesn't make the switch forward them any faster.

The only time a frame with "EF" on it will be treated any differently is when there is congestion and the switch has to decide which frames to drop - the frames without "EF" on them will be dropped first. If you've marked too much stuff with "EF", then your switch will start dropping those frames as well. This is why "EF" is normally used only for voice: voice doesn't use much bandwidth so whatever happens, QoS should ensure none of it is ever dropped.

Your 2nd question is right on target - you need to ensure your switch is set to "trust" the frames coming into the switchport - check your QoS settings (I've never used a 1920) and see if there is a "Trust" option. 

You might have two choices: trust Layer2 (COS) or trust Layer3 (DSCP). In your case, "EF" is DSCP.

[PiotrIr]

Many thanks for your clear reply.

I found where to set Trust on ports so this does make perfect sense for me. Is it enought if I will enable trust on all ports or do I need to set setting on WRR as well? When I check queue default settings I can see as below. So just wander if does it mean DSCP 46 will be translated to QID 5 but as it has the same weight as QID 0 it will make no difference at all or I missed something?

QType                                       QID                               Group                                    Weight

WRR                                            0                                      1                                               1 

WRR                                            1                                      1                                               1 

WRR                                            2                                      1                                               1 

WRR                                            3                                      1                                               1 

WRR                                            4                                      1                                               1 

WRR                                            5                                      1                                               1 

WRR                                            6                                      1                                               1 

WRR                                            7                                      1                                               1

[Vince-Whirlwind · Accepted solution]

It might look like it makes no difference, but think about this:

 1 phonecall = 100Kb/s

 1 file transfer = 600Mb/s

The packets in the phonecall get marked with DSCP 46 and are assigned to Queue5 with a weight of 1

The packets in the file transfer get marked with DSCP of 0 and are assigned to queue0 with a weight of 1

The switch now devotes the *same* amount of resources to switching the VoIP packets as they do to switching the other packets.

In other words, the switch will be forwarding all VoIP packets as soon as it is already forwarding 0.02% of the file transfer packets.

You can change the "weight" if you like, but don't bother unless you've added up your bandwidth requirements and you have a specific goal in mind.

[PiotrIr]

Vince,

Thank you for your reply, it is very dificault to find someobody who knows what he is talking about - and you know how this works.

Thank you.