Refer to the exhibit. This Layer 2 ring has 10 VLANs with 1000 MAC addresses in each VLAN. Which protocol or mechanism provides the shortest traffic outage if the link marked with "X" fails?

Question

Exhibit

Options

• AEthernet linear protection switching
• BPVRST
• CMST
• DEthernet ring protection switching

• A
  15% (10)
• B
  3% (2)
• C
  9% (6)
• D
  73% (49)

Explanation

Ethernet Ring Protection Switching (ERPS/G.8032) is purpose-built for ring topologies and achieves failover in under 50 milliseconds by pre-blocking one link (the RPL) and immediately unblocking it upon failure detection - no reconvergence calculation needed.

Why the distractors fall short:

• A. Ethernet Linear Protection Switching (G.8031): Designed for point-to-point links, not rings. It offers fast protection (~50ms) but doesn't match ERPS's efficiency in a ring topology where the failure model is fundamentally different.
• B. PVRST (Per-VLAN Rapid Spanning Tree): With 10 VLANs and 1000 MACs each, PVRST runs a separate spanning tree instance per VLAN, adding overhead. Convergence is typically 1–5 seconds - far slower than ERPS.
• C. MST (Multiple Spanning Tree): Better than PVRST at scale by grouping VLANs into instances, but still STP-based convergence (1–5 seconds). Neither STP variant was designed for ring topologies.

Memory tip: Think of ERPS as a "pre-loaded spring" - the RPL port is already blocked, so when the link fails, the ring simply snaps open in one direction rather than having to recalculate the whole tree. "Ring = ERPS, Line = Linear, Tree = STP" maps protocol to topology cleanly for exam day.

No community discussion yet for this question.