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Routing & Switching · Foundational

Class of Service (CoS)

The 3-bit Priority Code Point in the 802.1Q tag used for Layer 2 QoS marking (IEEE 802.1p).

Definition

CoS values 0-7 exist only on tagged frames, so they are lost when frames are routed or sent untagged. Voice typically uses CoS 5 and call signaling CoS 3. Switches map CoS to DSCP at trust boundaries. Junos uses the term class of service for its overall QoS framework.

Example

mls qos trust cos on an uplink of an older Catalyst switch

Exam tip

CoS is Layer 2 and lives in the 802.1Q tag; DSCP is Layer 3 and survives routing.

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