IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

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IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

TCP MSS adjustment

Updated: August 12, 2026

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Explains how TCP MSS adjustment modifies the maximum segment size of transit IPv4 and IPv6 TCP SYN and SYN-ACK packets to keep flows within the interface MTU and prevent fragmentation during session establishment.

TCP maximum segment size (MSS) adjustment is a traffic management mechanism that

  • modifies the maximum segment size value on transit IPv4 and IPv6 packets

  • supports TCP SYN and TCP SYN-ACK packets on configured interfaces, and

  • helps prevent fragmentation by keeping TCP flows below the interface MTU.

Fragmentation prevention

TCP MSS adjustment modifies the maximum segment size on transit IPv4 and IPv6 TCP SYN and TCP SYN-ACK packets. This behavior helps ensure that TCP segments remain within the interface MTU during session establishment.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

TCP MSS adjustment

Release 26.2.1

Introduced in this release on: Fixed Systems (8200, 8700, 8010)(select variants only*), Modular Systems (8800 [LC ASIC: P100])(select variants only*)

You can now prevent packet fragmentation and ensure TCP segments remain within the interface MTU by using TCP MSS adjustment. This feature modifies the Maximum Segment Size on transit IPv4 and IPv6 TCP SYN and TCP SYN-ACK packets. This helps ensure optimal traffic flow during session establishment.

*This feature is now supported on:

Line cards:

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Fixed systems:

  • 8212-48FH-M

  • 8011-4G24Y4H-I

  • 8011-32Y8L2H2FH

  • 8011-24Y8L2FH-I

  • 8011-24X

  • 8011-12G12X4Y

  • 8711-32FH-M

  • 8711-48Z-M

  • 8712-MOD-M

Guidelines and restrictions for TCP MSS adjustment

Use these guidelines and restrictions before you configure TCP MSS adjustment.

Guidelines for TCP MSS adjustment

  • Use TCP MSS adjustment on these supported interface types:

    • GRE tunnel interfaces

    • VLAN subinterfaces on physical Ethernet interfaces

  • Configure ipv4 tcp-mss-adjust enable , ipv6 tcp-mss-adjust enable , or both commands on an interface. Any of these configurations has the same effect.

  • The feature applies to TCP SYN and TCP SYN-ACK packets that are encapsulated in IPv4 or IPv6 frames.

Restrictions of TCP MSS adjustment

  • Use TCP MSS adjustment for transit IPv4 and IPv6 packets only.

  • Configure TCP MSS adjustment on the egress interface. If you configure TCP MSS adjustment on the ingress interface, the feature does not work.

  • Configure only one TCP MSS adjustment value per network processor (NP).

  • To apply the same TCP MSS adjustment value across all locations and all NPs simultaneously, use the hw-module command with the location all and np all options.

Configure TCP MSS adjustment

Configure TCP MSS adjustment to keep transit TCP flows below the GRE tunnel interface or VLAN subinterface MTU and prevent fragmentation during session establishment.

You can configure TCP MSS adjustment for IPv4 packets, IPv6 packets, or both on the egress interface.

Procedure

  1. Enable TCP MSS adjustment for IPv4 packets, IPv6 packets, or both on the egress interface.

    Example:

    Router# configure
Router(config)# interface GigabitEthernet 0/2/0/0.100
Router(config-if)# ipv4 tcp-mss-adjust enable
Router(config-if)# ipv6 tcp-mss-adjust enable
Router(config-if)# commit

  2. Exit interface configuration mode and configure the TCP MSS value for a network processor.

    Example:

    Router(config-if)# exit
Router(config)# hw-module location 0/0/CPU0 tcp-mss-adjust np 1 value 1300
Router(config)# commit

  3. Alternatively, apply the same TCP MSS adjustment value across all locations and all network processors.

    Example:

    Router(config-if)# exit
Router(config)# hw-module location all tcp-mss-adjust np all value 1300
Router(config)# commit