Basic QoS

[ryancisco01]

Hello,

I have two switches (L3), one at site A and one at site B, between them is a WAN link that is not QoS aware but has dedicated bandwidth, so all I need to do is order my packets appropriately.

My application tags my traffic already, all I want to do is prioritise the traffic so i send the higher priority out before the lower priority.

I understand on my input interfaces I just run this command: "mls qos trust dscp"

what command do I run on the WAN interface? if I run the same command (trust dscp) will the interface actually send the packets in order of priority or just persevere the tags? 

[Georg Pauwen]

Hello,

which switch models do you have, and what Ios versions are you running ?

[ryancisco01]

sorry yes that would have helped.

Its 3750X,  v12.2(55) on site A, site B is 3850 03.07.04e

[ryancisco01]

Anyone ale to assist with this? Was hoping its pretty simple

[Georg Pauwen]

Hello,

on the 3750X you typically use SRR. Each interface has 4 queues, and the default queue to DSCP mappings are:

Queue 1: DSCP 40 to 47
Queue 2: DSCP 0 to 16
Queue 3: DSCP 16 to 31
Queue 4: DSCP 32 to 39, 48 to 63

You can assign different bandwidth weights to each queue.e.g.:

srr-queue bandwidth shape 40 20 20 20

If you know exactly which DSCP values need priority, you can map the weights accordingly. I would suggest to configure the below on the WAN interface of your 3750X:

interface GigabitEthernet0/0

description WAN Link to 3850

srr-queue bandwidth shape 40 20 20 20

priority-queue out

The 'priority-queue out' simply means that queue 1 is always served first.

On the 3850, you could use something like the example below:

policy-map SHAPE_WAN
class-map DSCP
match dscp ef af41
shape average percent 50
class class-default
bandwidth remaining 50
fair-queue

!

interface GigabitEthernet0/0

description WAN Link to 3750X

service-policy output SHAPE_WAN

[Jose Mauro]

Hi,

You use the "mls qos trust dscp" to classify the traffic. (based on the DSCP)

For output, you need to configure the "queue". Look this:

https://community.cisco.com/t5/telepresence-and-video/help-understanding-qos-threshold/td-p/1374101

P.S. depending on the model of the switch the command changes due to queues and threshold, for example, on slide 47 shows that the 2960 switch queue is 1P3Q3T (1 priority, 3 queue, 3 threshold)

https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2019/pdf/BRKCRS-2501.pdf

[Joseph W. Doherty]

What do you mean by "dedicated bandwidth"? Do you mean you have some guaranteed rate but is it less than the physical interface's? Or do you mean you have caps for different classes of traffic?

On a 3750, to use QoS you must enable it globally, and then you have 4 hardware egress queues per interface. You "map" CoS or ToS markings to a particular hardware egress queue. You can customize how the 4 egress queues share bandwidth, how/when they drop traffic, and other options. One you enable QoS, by default, ingress ToS will be reset. Using the "trust" command preserves ingress markings. You can also have an ingress policy.

I haven't worked with a 3850, but believe its QoS is on by default, is configured more like a router (e.g. it "trusts" by default), and believe you have eight egress queues you can use. Also believe you can use a CBWFQ policy for egress.

One important consideration, by default QoS only engages when there's physical interface congestion. When working with a circuit which has a bandwidth cap less than the physical interface's, you generally need to "shape" your traffic. On the 3750 there a command to do this to the physical interface but it's inexact. Not sure what the 3850 offers. (NB: Generally "LAN" switches have a "weak" QoS feature set.)

[paul driver]

Hello

Can you elaborate further on your topology regards these two sites, explain how are they connected together, what type of connection is being used on the wan interface ( l2/l3 p2p, vpls.etc..), are you suing any routers, dynamic routing ?