IPsec Tunnel Requirements
IPsec tunnels for Secure Access must have an MTU that is no larger than 1390 bytes. Secure Access automatically clamps the TCP MSS to 1350 or below.
Fragmented packets in underlay or overlay are dropped. Slightly larger MTU and MSS may work depending on your IPsec configuration. For more information about establishing tunnels to Secure Access, see the tunnel configuration topics found in Network Tunnel Configuration.