RSTP vs STP: New Port Roles Explained
Short answer
CD RSTP adds the Alternate port and the Backup port roles, which were not explicitly defined in traditional STP.
Question
Key Takeaways
STP only has Root, Designated, and Blocking. RSTP replaces Blocking with two distinct roles: Alternate (backup for Root) and Backup (backup for Designated).
Learn which port roles are unique to RSTP compared to STP. The correct answers are Alternate and Backup ports, which enable faster convergence in network topologies.
Common mistake: Selecting only 'Alternate' or only 'Backup'. Students often forget that RSTP introduces *two* new roles, not just one. Alternatively, selecting Root/Designated is wrong because those existed in STP.
Expert Analysis
RSTP Introduces Two New Port Roles
Compared to the original IEEE 802.1D Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP) refines port states and adds specific roles to accelerate convergence. While STP only defines Root, Designated, and Blocking (non-designated/non-root) states, RSTP explicitly distinguishes between two new roles: Alternate and Backup.
The Alternate Port provides an alternative path to the root bridge. In STP, a port that is neither Root nor Designated is simply "Blocking." RSTP gives this role a specific name—Alternate—to indicate it is a backup for the Root Port. It listens for BPDUs but does not forward traffic until the primary path fails.
The Backup Port serves as a backup for a Designated Port. This occurs when a switch has multiple connections to the same shared segment or link (e.g., via a hub). If the primary connection to the segment fails, the Backup Port transitions quickly to forwarding. Like the Alternate port, the Backup port is in a discarding state under normal conditions.
Why Other Options Are Incorrect
- Root Port (A) and Designated Port (B) exist in both STP and RSTP; they are not new additions.
- Comments suggesting only one answer is correct often miss that RSTP adds both Alternate and Backup roles. The question asks "which... does RSTP add," implying identification of the new roles introduced in the standard.
Therefore, the correct choices are the ports that did not have explicit roles in STP: Alternate and Backup.
Exam Strategy
When comparing protocol versions, always list the base features first. For STP/RSTP, memorize the 3 STP roles (Root, Designated, Blocking) and then identify what replaced or refined them. RSTP's key innovation is splitting the non-forwarding state into Alternate and Backup to allow rapid transition without waiting for timers.
Frequently Asked Questions
What is the difference between an Alternate and a Backup port in RSTP?
Do Root and Designated ports exist in both STP and RSTP?
Community Votes
60% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 7 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 7
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GaboK 👍 3 Answered DAlternate port already exists in STP. The new port roles are Backup port and Edge Port.
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Silveira 👍 3 Answered CA correta é (C)
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IBB90704 👍 1Respuesta C y D Alternate Port: This is a new role introduced in RSTP. An alternate port is one that provides an alternative path to the root switch, different from the path that the root port takes. It's in a blocking state but can transition to forwarding quickly if the network topology changes and the primary path (via the root port) fails. Backup Port: This is another new role introduced in RSTP. A backup port provides a redundant (backup) path to a segment where another switch port already connects. This port is also in a blocking state, and will only become active if the designated port for that segment
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FerioSiqueira 👍 1 Answered CC and D Compared to Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP) introduces two additional port roles: Alternate Port: This port receives BPDUs (Bridge Protocol Data Units) from its own switch but remains in a blocked state unless the designated port fails. In STP, there's no equivalent role, and only blocked ports receive BPDUs from other switches. Backup Port: This port receives BPDUs from other switches but remains in a blocked state unless both the root port and designated port fail. This also doesn't have a direct equivalent in STP, offering an extra layer of redundancy.
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Fmoura17 👍 1 Answered CC and D are correct. STP vs RSTP Port Roles: Port RoleSTPRSTPRoot Port✅✅Designated Port✅✅Alternate Port❌✅Backup Port❌✅
Show 2 more comments
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Heidorn 👍 1 Answered DBackup Ports, normally STPhas Root and Designated ports, everything else becomes a alternate port.
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Chleong 👍 1DDDDDD
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