Why is MTU important in SD-WAN tunnels?

• A. MTU controls encryption strength.
• B. An improper MTU can cause fragmentation, increased retransmissions, and degraded performance; consistent MTU across tunnels minimizes fragmentation and avoids drops.
• C. An improper MTU only affects wireless networks.
• D. MTU is unrelated to SD-WAN performance.

Answer: (B)

MTU determines the largest packet size that can cross a link without being fragmented. In SD-WAN, traffic is carried through tunnels that add extra headers (like IPsec or other encapsulation), so the effective overhead reduces the usable space for the original payload. If the MTU is set too large for any tunnel path, packets will need to be fragmented or dropped, which leads to more retransmissions, higher latency, and overall worse performance. By choosing an MTU that fits the smallest path and keeping it consistent across all tunnels, fragmentation is minimized and packet drops are avoided, delivering smoother and more reliable SD-WAN performance.