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5.7.2. PHB (Per-Hop Behavior)
DSCP (Differentiated Services Code Point) is the standard way to mark IP packet priority. It's a 6-bit field in the IP header, giving 64 possible values (0-63). But the exam focuses on the common values:
| PHB | DSCP Value | Traffic Type | Treatment |
|---|---|---|---|
| EF (Expedited Forwarding) | 46 | Voice (RTP) | Low latency, low jitter, guaranteed bandwidth |
| AF (Assured Forwarding) | 10-43 | Business apps, video | Guaranteed delivery under normal conditions |
| CS (Class Selector) | 8, 16, 24... | Backward compatible | Maps to old IP Precedence |
| BE (Best Effort) | 0 | Everything else | No guarantees, drop first |
Layer 2 vs Layer 3 QoS:
- CoS (Class of Service): 3-bit field in 802.1Q VLAN tag (0-7)—only exists on trunks
- DSCP: 6-bit field in IP header—survives across the entire path
⚠️ Exam Trap: CoS marks are lost when a trunk connects to an access port. DSCP marks survive end-to-end. For consistent QoS across the network, trust DSCP at network edges and map between CoS and DSCP at trunk boundaries.
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Written byAlvin Varughese
Founder•18 professional certifications