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5.7.1. QoS Mechanisms
| Mechanism | What It Does | Analogy |
|---|---|---|
| Classification | Identify traffic types (ACL, NBAR, port numbers) | Check passenger's ticket |
| Marking | Tag packets with priority (DSCP, CoS) | Stamp "PRIORITY" on boarding pass |
| Queuing | Put packets in different lines based on marks | Multiple security lines |
| Congestion avoidance | Proactively drop low-priority before queue fills | Close regular line when busy |
| Policing | Hard rate limit—drop excess immediately | "No more passengers" |
| Shaping | Soft rate limit—buffer excess for later | "Wait in holding area" |
The QoS pipeline: Traffic flows through these stages in order. First, classify it (what kind of traffic is this?). Then mark it (attach a priority label). Then queue it (put it in the right line). Finally, transmit based on queue priorities.
Policing vs Shaping—when do you use each?
- Policing: Enforces contracts. ISPs police customer traffic at the edge—exceed your limit, excess gets dropped.
- Shaping: Smooths traffic. You shape outbound traffic to avoid overwhelming a slow WAN link.
Where drops appear under congestion: a priority (LLQ) class is always served first, up to its configured rate. When the egress link is congested, the other classes, including class-default (which catches everything unmatched), share the bandwidth that is left, and once their queues fill new packets are tail-dropped. show policy-map interface shows packet and drop counters per class.
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Written byAlvin Varughese
Founder•20 professional certifications