Question

A company runs an application on Compute Engine VMs in a VPC configured with an MTU of 1460. The VMs communicate with an on-premises data center over an HA VPN tunnel. Users report that small requests succeed, but large file transfers and certain database queries hang or time out intermittently. Ping with default packet sizes works fine between the VMs and on-premises hosts. What is the MOST likely cause and the correct action to resolve it?

Accepted Answer

Packets larger than the VPN tunnel's supported MTU are being dropped because the payload exceeds the overhead-adjusted limit; lower the MTU/MSS on the endpoints or enable MSS clamping so TCP segments fit within the tunnel.

Answer

The VPC firewall is blocking large packets; add an ingress rule permitting packet sizes above 1400 bytes.

Answer

BGP on the Cloud Router is advertising incorrect routes; reset the BGP session to re-learn the on-premises prefixes.

Answer

VPC Flow Logs sampling is too low to capture the large transfers; increase the sampling rate to 1.0 to restore connectivity.