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Routing & Switching · Intermediate

Per-Hop Behavior (PHB)

The forwarding treatment a DiffServ router applies to packets based on their DSCP marking.

Definition

A PHB describes externally observable treatment such as priority, bandwidth share and drop preference, not a specific queuing implementation. Standard PHBs are Default (best effort), Class Selector (backward compatible with IP Precedence), Assured Forwarding (four classes with three drop precedences) and Expedited Forwarding (low loss, latency and jitter).

Example

Routers map EF to a strict priority queue.

Exam tip

PHB is the behavior; DSCP is the marking that selects it.

Related terms