QoS per-hop behavior
Classification, marking, queuing, congestion management and policing versus shaping — the per-hop forwarding behaviors QoS defines.
QoS defines per-hop behavior: what each device does to a packet as it forwards it, hop by hop — that's the DiffServ model — per-hop, with no built-in end-to-end reservation (reservation-oriented approaches like IntServ/RSVP are a different model). The chain is: classify (recognize traffic) → mark (tag it for downstream trust) → queue/schedule (who goes first under congestion) → optionally police/shape (enforce rates).
QoS mechanisms
| Mechanism | What it does | Where it acts |
|---|---|---|
| Classification | Identify traffic by ACL, port, DSCP, NBAR | Ingress |
| Marking | Write DSCP (L3) or CoS (802.1p, L2 tag) into the packet | Ingress — closest to the source |
| Trust boundary | Where you accept vs rewrite arriving marks | Network edge/access ports |
| Congestion management (queuing) | Order packets for transmission — e.g., priority queue for voice | Egress, when the link is full |
| Congestion avoidance | Drop early to prevent tail-drop meltdown (WRED) | Egress before buffers fill |
| Policing | Enforce a rate — excess is dropped or re-marked | Ingress or egress |
| Shaping | Enforce a rate — excess is buffered/smoothed | Egress — delays rather than drops |
Marking values to recognize
| Field | Where | Examples |
|---|---|---|
| DSCP | IPv4/IPv6 header (6 bits) | EF (voice), AF41/AF31 classes, CS0/BE default |
| CoS (PCP) | 802.1Q tag (3 bits) | 0–7; 5 conventionally voice |
DSCP travels end-to-end in the IP header; CoS exists only inside an 802.1Q tag — on interswitch trunks and on tagged voice/edge frames alike.
Policing measures traffic against a configured rate and drops or re-marks the excess — it does not delay. Shaping buffers excess traffic to emit a smooth rate — it delays instead of dropping but can still drop once buffers fill. Voice is delay-, jitter- and loss-sensitive, so the usual pattern is a parent WAN shaper with a child priority queue rather than shaping alone; providers police customers, enterprises shape to their contracted rate.
Related resources
Sources
- Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
- RFC 2474 — Definition of the Differentiated Services Field (DS Field) — reviewed 2026-11-02
- RFC 4594 — Configuration Guidelines for DiffServ Service Classes — reviewed 2026-11-02
Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.