What steps would you take to debug an SD-WAN tunnel that shows high latency but low packet loss?

• A. Check underlay latency, MTU issues, QoS shaping, path quality scores, compare with other tunnels, and inspect cloud/SaaS path affecting performance
• B. Reboot the edge devices and reinitialize tunnels
• C. Increase the MTU to 9000 on all tunnels
• D. Disable QoS to reduce processing

Answer: (C)

High latency with no packet loss usually means the packets are getting through, but something in the tunnel or underlay is adding delay rather than dropping data. A common contributor is MTU alignment. If end-to-end MTU isn’t consistently supported, packets may be fragmented or require extra processing as they traverse the network, which can add latency. Testing with a jumbo MTU, such as 9000, on SD-WAN tunnels can help you see whether reducing fragmentation and per-packet processing lowers latency. If the path truly supports 9000 end-to-end, you may observe a latency improvement; if not, you’ll note that jumbo frames aren’t beneficial for this path and can even cause issues. Before making such a change, verify the path MTU across the underlay and ensure all devices and cloud/SaaS egress points support the larger frame. In practice, you’d also examine underlay latency, QoS shaping, path quality scores, and how other tunnels perform to pinpoint whether the delay is tied to the MTU or another factor.