If shown, the four switches have run the RSTP protocol and completed spanning tree calculations. So which of the following descriptions of port roles are correct? (Multiple choice)

Question

Options

• AThe G0/0/1 port of SW2 is the BP port, and the G0/0/2 port is the AP port.
• BThe G0/0/1 port of SW3 is the DP port, and the G0/0/2 port is the AP port.
• CThe G0/0/1 port of SW4 is the RP port, and the G0/0/2 port is the AP port.
• DThe G0/0/1 port of SW2 is the AP port, and the G0/0/2 port is the BP port.

• A
  80% (16)
• B
  5% (1)
• D
  15% (3)

Explanation

Options A and C are correct because they accurately describe RSTP port roles based on each switch's position relative to the root bridge. In A, SW2's G0/0/1 is a Backup Port (BP) - meaning SW2 has two connections to the same LAN segment and this port is the redundant/inferior one; G0/0/2 is an Alternate Port (AP) - a blocked port that provides an indirect alternate path to the root. In C, SW4's G0/0/1 is a Root Port (RP) - the single best upstream path toward the root bridge - while G0/0/2 is an AP, blocked because another switch offers a superior path on that segment.

B is wrong because SW3's G0/0/1 is not a Designated Port in this topology - based on SW3's position relative to the root, it should be a Root Port, not a port forwarding on behalf of its segment toward downstream switches.

D is wrong because it simply reverses the roles stated in A - it assigns AP to G0/0/1 and BP to G0/0/2 on SW2, which contradicts the actual port costs and BPDU comparisons in the topology.

Memory tip: Distinguish BP vs. AP by the letter: Backup = Both ports on the same bridge reach the same segment (one backs up the other); Alternate = another different bridge is the designated forwarder on that segment, so this port is the alternate root path - blocked but ready to take over if the root port fails.

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