300-365 · Question #43
A network engineer is configuring QoS and sees a DSCP value of 40. To which CoS value will this be mapped?
The correct answer is C. 5. DSCP values map to CoS by taking the three most significant bits of the 6-bit DSCP field, which is equivalent to dividing the DSCP value by 8 and discarding the remainder.
Question
A network engineer is configuring QoS and sees a DSCP value of 40. To which CoS value will this be mapped?
Options
- A4
- B3
- C5
- D2
- E1
How the community answered
(20 responses)- B5% (1)
- C90% (18)
- D5% (1)
Why each option
DSCP values map to CoS by taking the three most significant bits of the 6-bit DSCP field, which is equivalent to dividing the DSCP value by 8 and discarding the remainder.
CoS 4 corresponds to DSCP values 32-39 where the upper three bits equal binary 100, not DSCP 40.
CoS 3 corresponds to DSCP values 24-31 where the upper three bits equal binary 011, not DSCP 40.
DSCP 40 in binary is 101000; the three most significant bits are 101, which equals decimal 5, yielding a CoS value of 5. This mapping follows the IEEE 802.1p derivation where the upper 3 bits of the DSCP value directly correspond to the 3-bit CoS field. All DSCP values from 40 to 47 map to CoS 5 under this standard formula.
CoS 2 corresponds to DSCP values 16-23 where the upper three bits equal binary 010, not DSCP 40.
CoS 1 corresponds to DSCP values 8-15 where the upper three bits equal binary 001, not DSCP 40.
Concept tested: DSCP to CoS value mapping using 3-bit derivation
Source: https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-packet-marking/10103-dscpvalues.html
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