Forwarding methods: Does a cut-through Ethernet switch read only the initial portion of the frame (for example, up to the destination MAC) and begin forwarding <em>before</em> the entire packet is received?

Introduction:
Ethernet switches commonly implement one of two forwarding modes: store-and-forward or cut-through. The distinction affects latency and error handling. This question checks whether you know that cut-through switching minimizes latency by starting transmission as soon as the destination address (and possibly VLAN tag) is parsed.

Given Data / Assumptions:

• We are discussing Ethernet switching behavior.
• Cut-through mode emphasizes low latency over full error checking before forward.
• Store-and-forward reads the entire frame and validates the FCS first.

Concept / Approach:
In cut-through, the switch forwards a frame as soon as it determines the egress port (typically after parsing the destination MAC and any necessary header fields). This reduces per-hop latency. By contrast, store-and-forward receives the full frame, verifies the FCS, and only then forwards—adding latency but improving error containment. Many devices support hybrids (fragment-free) that read a minimum portion to avoid forwarding collisions fragments.

Step-by-Step Solution:
1) Identify the forwarding decision point: destination MAC found early in the frame.2) Recognize that cut-through forwards immediately after this lookup.3) Contrast with store-and-forward, which waits for the full frame and FCS.4) Conclude that the statement “cut-through reads the entire packet first” is incorrect; the correct understanding is early forwarding.

Verification / Alternative check:
Latency measurements show cut-through switches forwarding within microseconds tied to header reception time rather than full frame duration.

Why Other Options Are Wrong:

• Store-and-forward claim: Describes the opposite mode.
• Jumbo/fiber/Token Ring qualifiers: Irrelevant to the fundamental definition of cut-through.

Common Pitfalls:
Assuming all switches always validate FCS first; that is specific to store-and-forward mode, not cut-through.

Final Answer:
Correct — cut-through forwards after reading the header, not the entire frame