N10-009 Question 23
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 cloud-based VoIP system securely over the internet. Users report that voice calls become choppy during large file transfers, and the network administrator also notices that packets sent across the path expire when a routing loop occurs during a failover test. Which action would BEST improve voice-call performance across the VPN while still relying on the packet field that prevents indefinitely circulating packets during loops?
- A
Configure QoS to prioritize real-time voice traffic over bulk data traffic across the WAN/VPN path
- B
Increase the TTL value on all client devices so voice packets remain on the network longer during congestion
- C
Disable TTL processing on the VPN tunnel so encrypted packets are not dropped during routing changes
- D
Replace the VPN with NAT so voice packets bypass encryption overhead and loops are reduced
Show answer and explanation
Correct answer: A
Explanation
This scenario combines three Network+ function topics: VPN, QoS, and TTL. The practical issue affecting users is poor VoIP quality during file transfers over a site-to-site VPN. The best remediation is QoS, because real-time applications such as voice are highly sensitive to latency, jitter, and packet loss, whereas bulk transfers can tolerate delay better. Industry best practice is to classify and prioritize voice traffic using QoS mechanisms such as traffic marking and queuing on WAN edge devices and other trusted network points. TTL, by contrast, is an IP header field used to prevent packets from looping forever; routers decrement it by one at each hop, and the packet is discarded when it reaches zero. This behavior is commonly observed during troubleshooting with routing loops or tools such as traceroute. VPNs provide secure transport across untrusted networks, but they do not inherently solve traffic prioritization problems. References aligned with common networking documentation include RFC 791 for IPv4 TTL behavior, RFC 2474 for differentiated services concepts used in QoS, and standard vendor best practices for prioritizing VoIP traffic over WAN/VPN links.
- A. Correct.
Correct. QoS is designed to classify and prioritize delay-sensitive traffic such as VoIP over less time-sensitive traffic like file transfers. In a real environment, administrators commonly mark or queue voice traffic to reduce latency, jitter, and packet loss. This is the best action to improve call quality across a VPN/WAN connection. The mention of packets expiring during loops refers to TTL, which already serves its intended purpose of preventing packets from circulating indefinitely.
- B. Incorrect.
Incorrect. TTL is not a congestion-management or voice-quality tuning mechanism. Time to live limits the number of Layer 3 hops a packet can traverse before being discarded, which helps prevent endless looping when routing problems occur. Raising TTL does not prioritize VoIP traffic or meaningfully solve choppy calls caused by contention with bulk transfers.
- C. Incorrect.
Incorrect. TTL processing is a fundamental IP behavior used by routers to decrement the packet's lifetime at each hop. Disabling TTL processing is not a normal or appropriate fix and would undermine loop prevention. The VPN does not eliminate the need for TTL; encrypted traffic still traverses routed networks where loop protection is required.
- D. Incorrect.
Incorrect. NAT is not a replacement for a VPN. A VPN provides secure encrypted connectivity, while NAT translates addresses. Replacing the VPN with NAT would remove confidentiality and integrity protections and would not address the core issue of prioritizing voice traffic. NAT also does not reduce routing loops; TTL is what limits the effect of loops.