Which is a commonly cited cause of SD-WAN tunnel flaps?

• A. Underlay instability
• B. Excessively long MTU values on user devices
• C. Disabled DNS service on a laptop
• D. Incompatible VPN client on a host

Answer: (A)

SD-WAN tunnel stability hinges on the reliability of the underlay network. When the underlying transport (the physical or logical paths carrying the traffic) is unstable—think intermittent link drops, jitter, or frequent path changes—the overlay tunnels that SD-WAN builds tend to repeatedly tear down and reestablish. Each rekey or tunnel restart shows up as a tunnel flap, disrupting traffic and triggering policy reevaluations. That’s why underlay instability is the most commonly cited cause.

The other scenarios don’t typically cause repeated tunnel flaps. Excessively long MTU values on user devices can lead to fragmentation and performance problems, but not frequent tunnel up/down events. Disabled DNS affects name resolution, not the tunnel state. An incompatible VPN client on a host could prevent a tunnel from forming, but it’s not a frequent, systemic cause of flapping across environments like underlay instability is.