DSCP markings decoded: EF, AF and CS without the guesswork

QoS lives or dies on marking traffic correctly. DSCP gives you 64 possible values, but in practice a small set of named markings covers almost everything you will configure — once you know what the names mean.

DSCP is six bits

DSCP is the top six bits of the IPv4 Type of Service byte (the IPv6 Traffic Class byte), which is why values run 0 to 63. The remaining bits handle explicit congestion notification. Those six bits are grouped into three families with standard meanings.

The three families

• →CS0 to CS7 (Class Selector) — map one-to-one to the legacy IP precedence values, so CS-only devices interoperate cleanly.
• →AF (Assured Forwarding), written AFxy — x is the queue class (1 to 4), y is the drop precedence (1 low, 3 high) within that queue.
• →EF (Expedited Forwarding), DSCP 46 — the strict-priority marking for real-time media.

The markings you will actually set

• →EF (46) — voice / RTP media
• →CS3 (24) — call signaling
• →AF41 (34) — interactive / real-time video
• →CS6 (48) — network control (routing protocols)
• →CS1 (8) — scavenger / bulk background

Mark once, at or near the source, then trust and queue on those markings across the core. Re-marking mid-path is where QoS designs usually go wrong.