Cisco Catalyst SD-WAN Network Configuration Guide, Releases 26.x and Later

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Cisco Catalyst SD-WAN Network Configuration Guide, Releases 26.x and Later

Restrictions for TCP MSS and clear dont fragment

Updated: April 24, 2026

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Describes limitations and restrictions associated with TCP MSS and clear don't fragment, outlining operational boundaries and unsupported scenarios for these features.

TCP MSS

  • TCP MSS values can be adjusted for Cisco Catalyst SD-WAN tunnel interfaces only.

    From Cisco IOS XE Catalyst SD-WAN Release 17.9.1aand Cisco vManage Release 20.9.1, you can adjust the TCP MSS value for a service VPN or for Network Address Translation (NAT) Direct Internet Access (DIA) use cases. Adjusting the TCP MSS value helps prevent TCP sessions from being dropped.

    For more information on NAT DIA, see the Cisco Catalyst SD-WAN NAT Configuration Guide.

  • From Cisco IOS XE Catalyst SD-WAN Release 17.18.1a, for an IPv4 SD-WAN tunnel carrying IPv6 traffic, the system uses the IPv6 TCP MSS value. Also, for an IPv6 tunnel carrying IPv4 traffic, the system uses the IPv4 TCP MSS value. This upgrade aligns MSS adjustment with the traffic protocol and tunnel configurations, ensuring accurate and efficient handling of traffic.

Clear dont fragment

  • The option Clear Dont Fragment is available for Cisco Catalyst SD-WAN tunnel interfaces only.