mls qos

Overview

The `mls qos` command is a fundamental global configuration command on Cisco Catalyst switches that enables the Multilayer Switching (MLS) Quality of Service (QoS) feature. When issued, it activates the switch's hardware-based QoS engine, allowing the device to classify, mark, police, and queue traffic based on Layer 2 (CoS), Layer 3 (IP precedence or DSCP), or other criteria. Without this command, the switch operates in a best-effort mode, treating all packets equally, which can lead to poor performance for delay-sensitive applications like voice and video.

This command is the first step in any QoS deployment on Catalyst switches; it must be enabled before configuring trust boundaries, shaping, policing, or queueing policies. The command is available in global configuration mode and does not require any additional parameters—simply typing `mls qos` turns on the QoS feature. Once enabled, the switch begins to honor the default trust settings (typically untrusted on access ports) and allows the configuration of more advanced QoS policies.

It is important to note that `mls qos` is a per-switch command; it affects all interfaces on the switch. The command is supported on most Catalyst switches running IOS, including the 2960, 3560, 3750, 3850, and 4500 series. On newer platforms running IOS-XE (e.g., Catalyst 9300), the equivalent command is `qos` under the global configuration, but `mls qos` is still accepted for backward compatibility.

The command does not have any parameters; it is a simple toggle. When you issue `no mls qos`, QoS is disabled globally, and all QoS configurations are removed from the running configuration. This command is typically used in enterprise networks where traffic prioritization is required, such as in VoIP deployments, video conferencing, or when implementing strict service-level agreements (SLAs).

It fits into the broader QoS workflow: first, enable `mls qos` globally; second, configure trust boundaries on interfaces (e.g., `mls qos trust cos`); third, define class maps and policy maps for classification and policing; fourth, apply service policies to interfaces. The command is also used in troubleshooting scenarios where QoS behavior is unexpected—checking if `mls qos` is enabled is often the first step. A common mistake is forgetting to enable `mls qos` before applying QoS policies; the policies will be accepted in the configuration but will not take effect until the global command is issued.

Another important behavior is that `mls qos` is saved in the running configuration and startup configuration; it persists across reloads. The command does not require any special privilege level beyond the standard 15 (enable mode). When enabled, the switch's QoS hardware resources are allocated, which may impact performance if too many policies are applied.

In summary, `mls qos` is the gateway to all QoS features on Catalyst switches and is essential for any network engineer deploying QoS in a campus or data center environment.