Exam 300-420 topic 1 question 133 discussion

Actual exam question from Cisco's 300-420

Question #: 133
Topic #: 1

A network engineer is redesigning a company's QoS solution. The company is currently using IP Precedence, but the engineer plans to move to DiffServ. It is important that the new solution provide backward compatibility with the current solution. Which technology should the design include?

A. expedited forwarding
B. assured forwarding
C. class selector code points
D. default per-hop behavior

Show Suggested Answer

Suggested Answer: C 🗳️

by namibdigger at Aug. 1, 2022, 8:56 a.m.

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Lungful

9 months, 3 weeks ago

Selected Answer: C

C is correct. "To preserve backward-compatibility with any IP precedence scheme currently in use on the network, DiffServ has defined a DSCP value in the form xxx000, where x is either 0 or 1. These DSCP values are called Class-Selector Code Points. (The DSCP value for a packet with default PHB 000000 is also called the Class-Selector Code Point.)" Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_dfsrv/configuration/15-mt/qos-dfsrv-15-mt-book/qos-dfsrv.html

upvoted 1 times

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Reinier_veen

1 year, 7 months ago

annoying that they use other terms. " class selector code points" means "Differentiated Services Code Point" (DSCP)?

upvoted 1 times

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namibdigger

1 year, 10 months ago

Selected Answer: C

FiffServ is backward compatible with IP Precedence (Ip-Precedence uses the 3 most signifcant bits of the ToS byte, whereas Diffserve uses the fost significant six bits - which includ the ones from IP precedence)

upvoted 4 times

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