auto qos - is it still required?

[andrew-kearton]

Hi,

Looking for some guidance/ideas.  I'm about to deploy 3850 and 3650 switches to a site, and I'm wondering if auto qos is really still required nowadays.  Back with the 3750s and other older access switches, the norm was to run up auto qos on edge switchports and uplink ports, because the default setting was to mark all CoS and DSCP down to 0 if the port was not trusted.

My understanding is that DSCP and CoS markings are now trusted by default on the newer switches, which leads to the question, is QoS/CoS configuration actually required on edge switchports anymore?  If not, then how best to handle uplink ports?

I guess I can just put the same old commands in, but I'm trying to simplify the configuration on the switches.  Also my understanding is once you run auto-qos, it "undoes" the trust across the entire switch meaning you have to put it on all edge switchports.

Most of the documentation I can find just explains how to implement the commands, whereas I want to know why/if I should.

Thanks in advance,

Andy

[Peter Paluch]

Andy,

If my memory serves me then unless you configured mls qos, the switch actually ignored the DSCP/CoS markings entirely, and did not rewrite them into 0 - but also did not honor any different QoS classes and treated all traffic as best effort. Only after mls qos was configured, ports were untrusted by default and that caused the DSCP/CoS markings to be zeroized. I do not have the documentation by me now but I would suppose that the 3850 behave in the same way.

Quite frankly, I do not believe that the auto-qos was ever truly needed. A port can be configured as trusted using mls qos trust per-interface command without calling up all the monstruous configurations that AutoQoS causes to be added. AutoQoS has been an attempt at "one shoe fits all" approach to configuring devices to provide decent QoS behavior but your mileage may vary.

It all boils down to a simple question with a not so simple answer: What exactly do you want to accomplish?

Best regards,
Peter

[andrew-kearton]

Thanks Peter.  It's a simple setup - 2x3850 switches as core with static routing to upstream WAN routers running HSRP (where the QoS settings are important).  Edge is 3650 stacks running LAN base connected back to the core with two 10gig ports (top and bottom of the stack).  Cisco phones on the edge with PCs off the back of the phone.

Really I just want to ensure that cos/qos markings at the edge make it through to the WAN routers unscathed.

[Peter Paluch]

Hi Andrew,

I will verify it for you till tomorrov (I am currently on travels) but based on my knowledge so far, it should be sufficient to set the ports as trusted using, ideally, mls qos trust dscp which will make sure that the markings are accepted and not rewritten.

I will update you here as soon as possible.

Best regards,
Peter

[Mark Malone]

Hi

hope you dont mind me jumping in here

i only did this recently on our network for our 3850s, mls qos was not available on our 3850s only auto qos or create your own class-maps policy etc , whats good though we went through it with TAC the switches trust Cisco phones by default no configuration was required , you can check this by using the command show platform qos dscp-cos counters gigabitEthernet x/x/x as below from one of my switches, the Cisco phone will mark at the source and the switch will carry it by default, dscp on ingress and egress we did no apply anything on our 3850s after this. Just to be sure we setup wireshark on each side of the switch and checked the packets to make sure packets from phones were marked with EF DSCP 46 and they were. The zero beside the values markings as per TAC ------Regarding to the second column it is completely normal that you see all 0 values.For next IOS this column will show the sum of  bytes for the packets matched with each DSCP value but now it is not implemented yet.

wan.sw1.interconnect#show platform qos dscp-cos counters gigabitEthernet 1/0/1

Ingress DSCP0 21776485364   0

Ingress DSCP1 1232408       0

Ingress DSCP2 93955752      0

Ingress DSCP3 4018          0

Ingress DSCP4 1194175999    0

Ingress DSCP5 10            0

Ingress DSCP6 2             0

Ingress DSCP7 3751          0

Ingress DSCP8 87076823      0

Ingress DSCP9 19            0

Ingress DSCP10 115202        0

Ingress DSCP11 1             0

Ingress DSCP12 5             0

Ingress DSCP13 0             0

Ingress DSCP14 1             0

Ingress DSCP15 0             0

Ingress DSCP16 258           0

Ingress DSCP17 0             0

Ingress DSCP18 64591685      0

Ingress DSCP19 0             0

Ingress DSCP20 0             0

Ingress DSCP21 1             0

Ingress DSCP22 0             0

Ingress DSCP23 0             0

Ingress DSCP24 286364821     0

Ingress DSCP25 1             0

Ingress DSCP26 16089585      0

Ingress DSCP27 1             0

Ingress DSCP28 3             0

Ingress DSCP29 0             0

Ingress DSCP30 0             0

Ingress DSCP31 0             0

Ingress DSCP32 1             0

Ingress DSCP33 0             0

Ingress DSCP34 103410714     0

Ingress DSCP35 0             0

Ingress DSCP36 3             0

Ingress DSCP37 0             0

Ingress DSCP38 0             0

Ingress DSCP39 0             0

Ingress DSCP40 26709605      0

Ingress DSCP41 0             0

Ingress DSCP42 5             0

Ingress DSCP43 0             0

Ingress DSCP44 0             0

Ingress DSCP45 2             0

Ingress DSCP46 389074085     0

Ingress DSCP47 5             0

Ingress DSCP48 33155783      0

Ingress DSCP49 1369          0

Ingress DSCP50 667148        0

Ingress DSCP51 10            0

Ingress DSCP52 1603          0

Ingress DSCP53 74            0

Ingress DSCP54 1180          0

Ingress DSCP55 20            0

Ingress DSCP56 5527          0

Ingress DSCP57 4021          0

Ingress DSCP58 44            0

Ingress DSCP59 7             0

Ingress DSCP60 6202          0

Ingress DSCP61 4370          0

Ingress DSCP62 38            0

Ingress DSCP63 7             0

Ingress COS0 24074613282   0

Ingress COS1 0             0

Ingress COS2 0             0

Ingress COS3 0             0

Ingress COS4 0             0

Ingress COS5 0             0

Ingress COS6 0             0

Ingress COS7 0             0

Egress DSCP0 33350262626   0

Egress DSCP1 719285        0

Egress DSCP2 406304379     0

Egress DSCP3 4654          0

Egress DSCP4 45988524      0

Egress DSCP5 0             0

Egress DSCP6 37            0

Egress DSCP7 3             0

Egress DSCP8 191448540     0

Egress DSCP9 0             0

Egress DSCP10 5416849714    0

Egress DSCP11 4             0

Egress DSCP12 25            0

Egress DSCP13 0             0

Egress DSCP14 1             0

Egress DSCP15 3             0

Egress DSCP16 2772719       0

Egress DSCP17 2             0

Egress DSCP18 817410367     0

Egress DSCP19 5             0

Egress DSCP20 560           0

Egress DSCP21 1             0

Egress DSCP22 113           0

Egress DSCP23 1             0

Egress DSCP24 908044757     0

Egress DSCP25 3             0

Egress DSCP26 34742743      0

Egress DSCP27 3             0

Egress DSCP28 4927          0

Egress DSCP29 2             0

Egress DSCP30 0             0

Egress DSCP31 2             0

Egress DSCP32 7281544       0

Egress DSCP33 1             0

Egress DSCP34 101106283     0

Egress DSCP35 3             0

Egress DSCP36 3             0

Egress DSCP37 0             0

Egress DSCP38 27122         0

Egress DSCP39 3             0

Egress DSCP40 84967830      0

Egress DSCP41 0             0

Egress DSCP42 38            0

Egress DSCP43 1             0

Egress DSCP44 15            0

Egress DSCP45 4             0

Egress DSCP46 761624833     0