300-320 Question #141: Real Exam Question with Answer & Explanation

The correct answer is C: transport reliability. Transport reliability refers to the inherent characteristics of the public internet as a transport medium: variable and often high latency, jitter (variable delay), and packet loss. Unlike a private WAN (MPLS, leased lines) where the service provider guarantees SLAs for these par

Question

A company uses site-to-site IPsec VPN tunnels established over the public internet as a backup to its private WAN. The interoffice IP telephony service quality is degraded when a failover event occurs. What known design limitation can cause this issue?

Options

Amaximum headend router throughput
Bcircuit bandwidth
Ctransport reliability
Dnumber of remote sites

Explanation

Transport reliability refers to the inherent characteristics of the public internet as a transport medium: variable and often high latency, jitter (variable delay), and packet loss. Unlike a private WAN (MPLS, leased lines) where the service provider guarantees SLAs for these parameters, the public internet offers no such guarantees. VoIP is extremely sensitive to latency (>150ms one-way is noticeable), jitter (>30ms causes choppy audio), and packet loss (>1% degrades quality significantly). When traffic fails over to the internet-based IPsec VPN, QoS markings set by the enterprise are typically ignored or re-marked by ISPs, and the path traverses unpredictable public infrastructure. This is a well-known and documented design limitation of using the public internet as a WAN backup for real-time traffic. Headend throughput (A), circuit bandwidth (B), and number of remote sites (D) may be concerns but are not the specific known limitation causing VoIP degradation on internet paths.

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