What is MQC?

Understanding MQC

MQC (Modular QoS CLI) is a Cisco IOS command-line framework for configuring Quality of Service in a consistent, reusable way. It separates QoS into three building blocks — class maps that match traffic, policy maps that define actions, and service policies that bind those actions to an interface — so the same definitions can be applied across many interfaces and devices.

It works in three sequential steps. First, a class-map identifies traffic using match criteria such as ACLs, DSCP markings, protocols (via NBAR), or input interface, with match-all or match-any logic. Second, a policy-map references each class and assigns QoS actions: marking (set dscp), policing (rate limiting), shaping, congestion management through queuing (CBWFQ, LLQ for voice), and congestion avoidance (WRED). Third, the service-policy command attaches the policy to an interface in the inbound or outbound direction, where it takes effect on live traffic.

It matters because QoS protects availability and performance of sensitive flows under congestion — the security pillar most often overlooked. Real-time traffic like VoIP and video collapses if it competes equally with bulk transfers, and uncontrolled traffic can be weaponized: policing and shaping configured through MQC can rate-limit or contain denial-of-service floods and bandwidth abuse, preserving capacity for critical services during an attack or surge.

For example, a network engineer creates a class-map matching voice packets marked EF, a class-map matching scavenger traffic, then a policy-map giving voice a low-latency priority queue while policing peer-to-peer traffic to a small ceiling. Applying that policy outbound on the WAN uplink with one service-policy command ensures calls stay clear during a backup window — and that a misbehaving host flooding the link cannot starve business-critical applications.