I am trying to better understand the default queuing behavior on Catalyst 3650/3860 running IOS XE 16.3.6 Denali.

  1. Is QoS enabled globally by default? Coming from working with Catalyst 2960X on IOS 15 I am familiar with the mls qos global configuration and the show mls qos command to determine status. I am having difficulty finding the analogous commands in IOS XE.
  2. Assuming an "out of the box" configuration on the above captioned hardware and IOS, what would be the behavior of IP packets with DSCP markings? Example: end devices are marking DSCP 46, 34, and 26 for three different traffic types respectively. Without service policies will these packets be treated differently than any other traffic?
  • QoS is enabled by default. You can enable Auto QoS version 4, by using auto qos srnd4 in global configuration. – user36472 Aug 2 '18 at 7:59
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    Here's a good basic overview of differences: cisco.com/c/en/us/support/docs/switches/… – user36472 Aug 2 '18 at 8:05
  • As a side note, in case you aren't aware already, Cisco announced the EoL of IOS XE 16.3.x Cisco IOS XE 16.3.x EoL Announcement You should consider using a different version. – Jesse P. Aug 3 '18 at 1:51
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    @JesseP. despite the fact, that the software is EoL, the 16.3.6 is still the suggested safe harbor software on Cisco download center. – user36472 Aug 3 '18 at 6:12

The configuration of QoS in the 3850 line has been improved due to its implementation of MQC (universal QoS configuration model) configuration instead of the old MLS QoS (platform-dependent QoS configuration) commands from the 3750 and 3560 lines of switches.

On the 3650/3850 series, QoS is enabled by default and all packets by default are trusted (the L2/L3 QoS marking is preserved), unless you change it with an application of a specific policy map on the ingress or egress interface.

There are two queues configured by default on each wired interface on the switch. All control traffic traverses and is processed through queue 0. All other traffic traverses and is processed through queue 1.

All routing control traffic in the network uses IP precedence value 6 by default. IP precedence value 7 also is reserved for network control traffic. Therefore, the use of IP precedence values 6 and 7 is not recommended for user traffic.

For wired or wireless ports that are connected to the switch (end points such as IP phones, laptops, cameras, telepresence units, or other devices), their DSCP, precedence, or CoS values coming in from these end points are trusted by the switch and therefore are retained in the absence of any explicit policy configuration.

So all in all, IP phones etc. are trusted and the DSCP/Precedence/CoS values retained, when nothing else has been configured.

Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/qos/configuration_guide/b_qos_3se_3850_cg/b_qos_3se_3850_cg_chapter_011.html

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