350-801 · Question #409
Which Layer 3 PHB marking is used for VoIP applications?
The correct answer is B. EF. Expedited Forwarding (EF) is the Layer 3 Per-Hop Behavior (PHB) marking specifically recommended and widely used for Voice over IP (VoIP) applications to ensure low latency, minimal jitter, and no packet loss.
Question
Which Layer 3 PHB marking is used for VoIP applications?
Options
- A0
- BEF
- CAF21
- DCS1
How the community answered
(44 responses)- A2% (1)
- B89% (39)
- C7% (3)
- D2% (1)
Why each option
Expedited Forwarding (EF) is the Layer 3 Per-Hop Behavior (PHB) marking specifically recommended and widely used for Voice over IP (VoIP) applications to ensure low latency, minimal jitter, and no packet loss.
A DSCP value of 0 (Best Effort) offers no QoS guarantees and is unsuitable for real-time VoIP traffic.
Expedited Forwarding (EF) is a well-defined Per-Hop Behavior (PHB) in Differentiated Services (DiffServ) architecture, marked by a DSCP value of 46. It is designed to provide a low-latency, low-jitter, and low-loss service, making it the ideal choice for real-time applications like VoIP that are highly sensitive to delay.
Assured Forwarding 21 (AF21) provides a medium-priority service with a lower drop precedence within the AF2x class, which is typically used for video conferencing or mission-critical data, but not the highest priority for pure voice.
Class Selector 1 (CS1) is primarily used for network control traffic or background traffic, not for high-priority, low-latency VoIP applications.
Concept tested: DiffServ PHB for VoIP
Source: https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_QoS/QoSDesignGuide/QoSDesignGuide.html
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