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Cisco Secure Access Help Network Tunnel Configuration Establish a Tunnel Maximum Transmission Unit (MTU) Size

Last updated: Aug 07, 2025

Maximum Transmission Unit (MTU) Size

The MTU determines the maximum packet size that can be sent over a network tunnel, thus setting an optimal MTU here is important. A suboptimal MTU for the tunnel results in significantly poor performance for your users.

An optimal tunnel MTU is equal to or lower than the following key values:

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.