Based on the figure, the forwarding of data traffic before roaming is STA->HAP->upper layer network, the forwarding method is direct forwarding, and the home agent is configured on the AP. After…

Question

Options

• ASTA -> FAP -> FAC -> HAC -> Upper-layer network.
• BSTA -> FAP -> HAC -> FAC -> Upper-layer network.
• CSTA -> FAP -> FAC -> Upper-layer network.
• DSTA -> FAP -> HAC -> HAC -> FAP -> Upper-layer network.

• A
  81% (22)
• B
  4% (1)
• C
  4% (1)
• D
  11% (3)

Explanation

Option A is correct because in Layer 3 roaming, the STA has moved to a different subnet managed by a Foreign AC (FAC). The data must travel from the Foreign AP (FAP) up to the FAC, which then tunnels the traffic back to the Home AC (HAC) - the controller that holds the home agent and maintains the STA's original IP context - before it can exit to the upper-layer network.

Why the distractors fail:

• B is wrong because it reverses FAC and HAC in the chain; traffic cannot bypass the FAC and jump directly from FAP to HAC, since FAC is the local controller that first receives and encapsulates the roamed traffic.
• C is wrong because it omits HAC entirely; direct forwarding from FAC to the upper network would be valid only if no L3 boundary had been crossed - after L3 roaming, the home agent (on HAC side) must be involved to maintain IP continuity.
• D is wrong because it lists HAC twice and routes traffic back through FAP before reaching the network, which is topologically nonsensical.

Memory tip: Think of L3 roaming as a "call forward" chain - traffic always flows outward through the local hierarchy first (STA → FAP → FAC), then tunnels homeward (FAC → HAC), and only then exits to the real network. The order mirrors the physical path: foreign first, home second, network last.

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