200-150 Question #105: Real Exam Question with Answer & Explanation

The correct answer is B: EtherType. Both the DSAP field in 802.3 LLC and the EtherType field in Ethernet II identify the upper-layer protocol that should receive the frame payload. EtherType is the Ethernet II equivalent of DSAP.

Cisco Data Center Networking Technologies

Question

Which field in an Ethernet II frame performs the same function as the DSAP field in an 802.3 Ethernet frame?

Options

AStart of frame
BEtherType
CFrame check sequence
DSubnetwork Access Protocol
ELogical Link Control

Explanation

Both the DSAP field in 802.3 LLC and the EtherType field in Ethernet II identify the upper-layer protocol that should receive the frame payload. EtherType is the Ethernet II equivalent of DSAP.

Common mistakes.

A. The Start of Frame delimiter (SFD) signals the beginning of the frame to receiving hardware - it plays no role in identifying the upper-layer protocol.
C. The Frame Check Sequence (FCS) is a 4-byte CRC trailer used for error detection, not for protocol type identification.
D. SNAP (Subnetwork Access Protocol) is an extension header used within 802.3 LLC frames to allow EtherType-style protocol identification, but it is not itself the counterpart to DSAP.
E. LLC (Logical Link Control) is the sublayer and header structure that contains DSAP and SSAP fields in 802.3 - it is the broader container, not the functional equivalent of DSAP.

Concept tested. EtherType vs DSAP protocol identification fields

Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/ethernet/17465-1.html

Topics

#Ethernet II frame#EtherType#802.3 frame#DSAP

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