N10-009 Question 22
Single answerFunctions: Virtual private network (VPN), Quality of service (QoS), Time to live (TTL)A company has deployed a site-to-site VPN between its headquarters and a branch office so employees can use a VoIP application hosted at headquarters. Users report that voice quality becomes poor during large file transfers, but the VPN tunnel remains up. A network administrator also notices that when testing connectivity across the path, packets with unusually low hop limits fail before reaching headquarters, while packets with normal hop limits succeed. Which action is the BEST way to improve the voice issue without changing the VPN design?
- A
Implement QoS policies that prioritize VoIP traffic over bulk data across the WAN/VPN path
- B
Increase the TTL value on all VoIP packets so they are processed before file-transfer traffic
- C
Replace the VPN with an unencrypted connection to reduce overhead and eliminate voice delay
- D
Lower the TTL on file-transfer packets so they are dropped earlier when congestion occurs
Show answer and explanation
Correct answer: A
Explanation
The key issue is that the VPN is operational, but real-time voice traffic is competing with bulk data transfers. The correct functional tool for this problem is QoS, which can classify and prioritize latency-sensitive traffic such as VoIP. TTL is often seen during troubleshooting with tools like ping and traceroute because it reveals whether packets can survive the number of routed hops in a path, but TTL does not provide service differentiation or bandwidth control. A low TTL failing while a normal TTL succeeds simply confirms the route length is greater than the lower test value. For Network+ objectives, candidates should recognize the distinct functions here: VPN provides secure connectivity over untrusted networks, QoS manages traffic priority and delivery characteristics, and TTL prevents packets from looping indefinitely across routers. Common vendor and standards documentation on QoS and IP behavior, including DSCP-based prioritization and IP TTL/hop limit handling, supports this distinction.
- A. Correct.
Correct. QoS is specifically designed to classify and prioritize delay-sensitive traffic such as VoIP over less time-sensitive traffic like file transfers. In this scenario, the VPN tunnel is functioning, but contention during large transfers is degrading call quality. Applying QoS on the WAN edge and ensuring voice traffic is properly marked and prioritized is the appropriate fix. This is a common best practice for converged networks carrying voice and data over the same path.
- B. Incorrect.
Incorrect. TTL (Time to Live) limits how many Layer 3 hops a packet can traverse before being discarded; it is not a prioritization mechanism. Raising TTL may allow packets to survive longer routes, but it does nothing to make voice traffic preferred over bulk transfers. The mention of low-hop-limit packets failing only indicates the path contains multiple routed hops, not that TTL tuning will solve congestion or latency-sensitive application performance.
- C. Incorrect.
Incorrect. Removing VPN encryption might reduce some overhead, but it creates a major security issue and does not address the core requirement of traffic prioritization. In real environments, site-to-site VPNs are commonly used over untrusted networks, and best practice is to keep the VPN and apply QoS where supported rather than eliminate encryption for convenience.
- D. Incorrect.
Incorrect. TTL is not intended for congestion management or selective dropping of lower-priority traffic. Lowering TTL on file-transfer traffic would simply make those packets expire after fewer routed hops, potentially breaking applications unpredictably rather than managing bandwidth fairly. Congestion and queue behavior should be controlled with QoS, shaping, or policing instead.