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.
The ENCOR blueprint has a specific, easily-underestimated line: "interpret QoS configurations." Not design them, not configure them from scratch, but read an existing policy and a show output and say what it does and whether it is
A single flat QoS policy works fine until reality intrudes: you have a 100 Mbps physical interface but the carrier only sold you 20 Mbps, or one physical link carries traffic for ten branch sites that each need their own
The obvious way to handle a full queue is to drop whatever arrives once the queue is full. It is called tail drop, it is the default, and it causes a subtle, damaging problem that gets worse exactly when the
When a WAN link congests, something has to give, and QoS is how you decide what. Left to itself, a link under load treats a voice packet exactly like a bulk file transfer, and the call breaks while the download
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.