STP Designated Port Election Parameters
Short answer
AB During STP designated port election, switches compare the Root Bridge ID advertised by peers and the Root Path Cost to determine the forwarding port for a segment.
Question
Key Takeaways
The question tests the hierarchical tie-breaking order of STP: Root ID first, then Root Path Cost. Candidates often confuse internal port IDs with external path metrics.
Learn which parameters are compared during STP designated port election. Correct answers are Root Bridge ID (A) and Root Path Cost (B).
Common mistake: Selecting D (Peer PID vs Local PID) is a common trap. PIDs are primarily used to break ties for ports on the *same* switch or to identify the sender in BPDUs, but the core election metrics between distinct network entities are the Root ID and Path Cost.
Expert Analysis
The STP designated port (DP) election process occurs on a per-segment basis to determine which switch port forwards traffic for that LAN. The algorithm follows a strict priority sequence to break ties.
First, switches compare the Root Bridge ID (R-BID). The switch advertising the lowest R-BID wins this step. This corresponds to the "Peer BID" concept in Option A, as each switch evaluates the root information received from peers to identify the current root.
Second, if multiple ports belong to the same Root Bridge (or bridges with identical priorities), they compare the Root Path Cost (RPC). The path with the lowest cumulative cost to the root is preferred. Option B correctly identifies this as a key comparison metric.
Third, to distinguish between two ports on the same switch connecting to the same segment (a loop scenario), the switch compares its own Local Port ID (PID). However, it does not directly compare the Peer PID against the Local PID across different switches; rather, it uses the Sender Port ID contained in the BPDU to evaluate the path. Therefore, comparing "Peer PID and local PID" (Option D) as a cross-switch election mechanism is technically incorrect. Option C refers to the internal logic of a single switch choosing which of its own ports becomes the DP.
In summary, the primary external parameters compared during the election hierarchy are the Root ID (from peers) and the Root Path Cost. The local Port ID is used only when necessary to differentiate ports on the same device.
Exam Strategy
When facing STP election questions, memorize the four-step tie-breaker sequence: 1. Lowest Root Bridge ID, 2. Lowest Root Path Cost, 3. Lowest Sender Bridge ID, 4. Lowest Sender Port ID. Always check if the question asks about the first two steps (Root ID and Cost) versus the later steps involving Bridge/Port IDs. For multi-select questions asking 'what is compared', prioritize the global metrics (Root ID, Cost) before local identifiers unless the scenario specifies a loop-back connection.
Frequently Asked Questions
Does STP compare the Port IDs of two different switches directly?
What happens if two switches have the same Root Bridge ID?
Community Votes
34% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 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 1
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HCIP Resolutor 👍 1RPC (Root Path Cost - Opción B): Es la segunda métrica clave evaluada tras comparar el Root ID. Se ignora por completo en la respuesta del dump. Peer PID and local PID (Opción D): STP NUNCA compara el PID del vecino directamente contra el PID local para decidir un Designated Port entre dos switches distintos. La comparación entre puertos de un mismo switch vecino solo usa el Sender Port ID (PID del puerto transmisor).
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