Quality of Service, the toolkit for keeping latency-sensitive traffic flowing predictably across crowded networks. Articles tagged QoS cover classification and marking (DSCP, CoS), MQC, queueing (LLQ, CBWFQ), policing, shaping, and voice and video treatment in SD-WAN and wireless.
ENCOR expects you to read an existing QoS policy and say what it does and whether it works. This post gives you a repeatable method for interpreting policy-maps and show output, using the real policy from the PingLabz QoS cluster.
A flat policy cannot say shape everything to 20 Mbps and give voice priority inside that 20. Hierarchical QoS nests one policy in another: a shaping parent and queueing children, built on Cisco IOS XE after the shaping vs policing groundwork.
Tail drop causes global synchronization exactly when the network is busiest. WRED fixes it by dropping packets early, deliberately, and at random. This guide explains why that works and how to configure it.
Build the full LLQ and CBWFQ queueing policy on Cisco IOS XE, then congest a real link to prove the priority class stays protected. Voice goes first, always, and everything else shares what is left fairly.
How to build a working QoS policy on a Cisco router: MQC class maps, policy maps, the LLQ + CBWFQ model, and the shaper rule that makes it actually engage.
End-to-end QoS for VoIP across an MPLS L3VPN: DSCP marking, the LLQ + CBWFQ policy at the CE edge, and the DSCP-to-MPLS-EXP mapping the carrier needs.
QoS in 2026 is application-aware steering at the SD-WAN edge plus local queueing per tunnel. Why DSCP stops being honored at administrative boundaries, and how SD-WAN path conditioning compensates.
Voice and video are the use cases QoS exists for. Latency and jitter budgets, DSCP markings, IP phone trust, LLQ configuration, codec bandwidth math, and wireless voice via WMM.
LLQ, CBWFQ, policing, and shaping each do something different. The three primitives, when to use which, hierarchical shaping for sub-rate WAN handoffs, and the production patterns.
Classification and marking at the trust boundary are the foundation of every QoS deployment. The patterns (untrusted access ports, conditional trust for IP phones, explicit MQC marking) and the discipline that makes them work.
The Cisco MQC three-construct model (class-map, policy-map, service-policy) covers every QoS feature on IOS, IOS XE, and IOS XR. The grammar, common actions, hierarchical policies, and verification.
DSCP is the 6-bit Layer 3 marking that drives modern QoS. The byte format, the standardized PHBs (CS, AF, EF), Cisco's DSCP-to-CoS-to-MPLS-EXP mapping, and trust boundary discipline.