RSTP defines four port roles. Among them, the Part role value of the Alternative port and the Backup port in the flags field of the STP packet is always 01.

Question

Options

• ATRUE
• BFALSE

• A
  74% (42)
• B
  26% (15)

Explanation

Option A is correct because in the RSTP BPDU flags field, two bits (bits 2–3) encode the port role, and both the Alternate port and the Backup port are assigned the binary value 01 - they share the same encoding even though they serve different functions in the topology. This design reflects that both roles are non-forwarding, agreeing ports that have been outcompeted by a better path or a superior port on the same segment.

Option B is wrong because the IEEE 802.1w standard explicitly defines the port role field encoding as: 00 = Unknown, 01 = Alternate/Backup, 10 = Root, 11 = Designated - leaving no ambiguity that 01 applies to both of these roles.

Memory tip: Think of the two non-forwarding "loser" roles (Alternate = lost root contest, Backup = lost segment contest) as sharing the same "consolation prize" tag of 01. The two forwarding winners - Root (10) and Designated (11) - each get their own unique, higher values.

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