350-401 · Question #11
Which two statements about marking fields are true? (Choose two)
The correct answer is C. The 3 priority bits are in 802.1Q/P. D. The IP DSCP field is in the IP header and is 6 bits long.. This question identifies true statements about common QoS marking fields, specifically focusing on their location and bit length.
Question
Options
- AThe IP Precedence field is in the IP header and is 4 bits long.
- BThe Frame Relay DE field is in the IP header and is 1 bit long.
- CThe 3 priority bits are in 802.1Q/P.
- DThe IP DSCP field is in the IP header and is 6 bits long.
- EThe ToS 6 bits are in the IP header.
How the community answered
(33 responses)- A3% (1)
- C94% (31)
- E3% (1)
Why each option
This question identifies true statements about common QoS marking fields, specifically focusing on their location and bit length.
The IP Precedence field is 3 bits long, not 4 bits, and is also part of the Type of Service (ToS) byte in the IP header.
The Frame Relay Discard Eligibility (DE) field is located in the Frame Relay header, not the IP header, and is 1 bit long.
The 802.1Q/P tag includes a 3-bit Priority Code Point (PCP) field, commonly referred to as Class of Service (CoS), which indicates the frame's priority level.
The Differentiated Services Code Point (DSCP) field is a 6-bit field located within the Differentiated Services (DS) byte of the IPv4 and IPv6 headers, used for packet classification and marking.
The Type of Service (ToS) byte itself is 8 bits long, and while DSCP (6 bits) is within it, stating 'ToS 6 bits' is imprecise when referring to the marking field.
Concept tested: QoS marking fields and their properties
Source: https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-packet-marking/100412-dscp-qos-ipv4-ipv6.html
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