MAC loopback and PHY loopback are used for fault location. When the MAC loopback is set to in loop, the PHY loopback can be set according to the actual situation.

Question

Options

• ATrue
• BFalse

• A
  17% (7)
• B
  83% (35)

Explanation

B is correct because MAC loopback and PHY loopback are mutually exclusive - when MAC loopback is set to "in loop," PHY loopback cannot be independently configured or enabled. The two loopback modes conflict because MAC loopback intercepts traffic at the data-link layer before it ever reaches the physical layer, making PHY loopback configuration meaningless or unavailable at that point.

Why A is wrong: The statement implies both loopbacks can be active or configured simultaneously "according to the actual situation," but this is not permitted. Network equipment enforces mutual exclusivity between these two modes precisely to prevent ambiguous fault isolation results - having both active at once would make it impossible to determine which layer has the fault.

Memory tip: Think of loopbacks as a layered funnel - traffic flows from MAC down to PHY. If you catch (loop) the traffic at the MAC layer, it never travels down to the PHY layer, so PHY loopback becomes irrelevant and unavailable. MAC up top catches it first - PHY never gets a shot.

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