BGP Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

TCP maximum segment size

Updated: February 28, 2026

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Overview

Key attributes of MSS
Per neighbor TCP MSS
Key attributes of per neighbor TCP MSS
Configure per neighbor TCP MSS
Disable the per neighbor TCP MSS
Summary of key commands for per neighbor TCP MSS

Overview

Describes TCP maximum segment size as a negotiated attribute that optimizes data transfer efficiency by controlling the largest unfragmented TCP segment size based on interface MTU.

Maximum Segment Size (MSS) is a TCP attribute that

determines the largest amount of data that a device can receive in a single, unfragmented TCP segment
is limited by the Maximum Transmission Unit (MTU) of an interface, and
is negotiated during the TCP setup process between a source and destination.

The MSS ensures efficient data transfer by optimizing the size of transmitted packets, especially for protocols like BGP. Each direction of data flow can use a different MSS value based on the MTU of the source and destination interfaces.

Key attributes of MSS

These are some of the key attributes of MSS:

The closer the MSS is to the MTU, the more efficient the data transfer.
The MSS is announced during the TCP setup process.
The default TCP MSS value is 536 octets or 1,460 bytes. This means that TCP segments the data in the transmit queue into 1460-byte chunks before passing the packets to the IP layer.

Per neighbor TCP MSS

Per neighbor TCP MSS is a mechanism in BGP configuration that

allows creating unique TCP MSS profiles for each neighbor
supports configuration in two modes: neighbor group and session group, and
overrides the global TCP MSS setting for specific neighbors.

Key attributes of per neighbor TCP MSS

These are some of the key attributes of per neighbor TCP MSS:

You can enable or disable TCP MSS configuration for specific neighbors.
MSS value can be reset to its default using the inheritance-disable command.
The configuration range for MSS values is from 68 to 10,000.

Configure per neighbor TCP MSS

The purpose of this task is to configure a TCP MSS value for a specific neighbor in BGP.

Before you begin

Identify the desired MSS value.

Procedure

	1.	Enter BGP configuration mode and set up the neighbor group. Example: `Router# configure Router#(config)# router bgp 10 Router#(config-bgp)# address-family ipv4 unicast Router#(config-bgp-af)# exit Router#(config-bgp)# neighbor-group n1 Router#(config-bgp-nbrgrp)# tcp mss 500 Router#(config-bgp-nbrgrp)# address-family ipv4 unicast Router#(config-bgp-nbrgrp-af)# exit Router#(config-bgp-nbrgrp)# exit`
	2.	Configure a specific neighbor and inherit settings from the neighbor group. Example: `Router#(config-bgp)# neighbor 10.0.0.2 Router#(config-bgp-nbr)# remote-as 1 Router#(config-bgp-nbr)# use neighbor-group n1 Router#(config-bgp-nbr)# address-family ipv4 unicast Router#(config-bgp-nbr-af)#`
	3.	Save the configuration. Example: `Router#(config-bgp-nbr-af)# commit`
	4.	Verify the running configuration on the system. Example: `Router# show running-config router bgp 10 address-family ipv4 unicast ! neighbor-group n1 tcp mss 500 address-family ipv4 unicast ! ! neighbor 10.0.0.2 remote-as 1 use neighbor-group n1 address-family ipv4 unicast ! ! ! end`
	5.	Use the show bgp neighbor command to view the neighbor configuration and status. Example: Router# show bgp neighbor 10.0.0.2 BGP neighbor is 10.0.0.2 Remote AS 1, local AS 10, external link Remote router ID 0.0.0.0 BGP state = Idle (No best local address found) ... Minimum time between advertisement runs is 30 secs Configured TCP Maximum Segment Size 500 Inbound message logging enabled, 3 messages buffered Outbound message logging enabled, 3 messages buffered ... For Address Family: IPv4 Unicast BGP neighbor version 0 Update group: 0.1 Filter-group: 0.0 No Refresh request being processed eBGP neighbor with no inbound or outbound policy; defaults to 'drop' ...
	6.	Use the show tcp brief command to check TCP connection endpoints. Example: `Router# show tcp brief PCB VRF-ID Recv-Q Send-Q Local Address Foreign Address State 0x000055e27958c800 0x60000000 0 0 :::179 :::0 LISTEN 0x000055e27958b850 0x00000000 0 0 :::179 :::0 LISTEN 0x00007f2a80002050 0x60000000 0 0 0.0.0.0:179 0.0.0.0:0 LISTEN 0x00007f2a840380d0 0x00000000 0 0 0.0.0.0:179 0.0.0.0:0 LISTEN` This information helps verify the correct configuration and troubleshoot connectivity issues. Use the show tcp command to view detailed TCP connection information.

The TCP MSS is configured for the specified neighbor.

Disable the per neighbor TCP MSS

The purpose of this task is to disable the TCP MSS configuration for a specific neighbor.

Follow these steps to disable the per neighbor TCP MSS:

Before you begin

Ensure the neighbor group or session group is already configured.

Procedure

	1.	Enter BGP configuration mode. Example: `Router# configure Router#(config)# router bgp 10 Router#(config-bgp)#`
	2.	Disable MSS inheritance for the neighbor group. Example: `Router#(config-bgp)# address-family ipv4 unicast Router#(config-bgp-af)# exit Router#(config-bgp)# neighbor-group n1 Router#(config-bgp-nbrgrp)# tcp mss inheritance-disable Router#(config-bgp-nbrgrp)# address-family ipv4 unicast Router#(config-bgp-nbrgrp-af)# exit Router#(config-bgp-nbrgrp)# exit`
	3.	Configure a specific neighbor and disable MSS inheritance for the neighbor. Example: `Router#(config-bgp)# neighbor 10.0.0.2 Router#(config-bgp-nbr)# remote-as 1 Router#(config-bgp-nbr)# use neighbor-group n1 Router#(config-bgp-nbr)# tcp mss inheritance-disable Router#(config-bgp-nbr)# commit`
	4.	Verify the running configuration on the system. Example: `Router# show running-config router bgp 10 address-family ipv4 unicast ! neighbor-group n1 tcp mss inheritance-disable address-family ipv4 unicast ! ! neighbor 10.0.0.2 remote-as 1 use neighbor-group n1 tcp mss inheritance-disable address-family ipv4 unicast ! ! !`

TCP MSS is disabled for the specified neighbor.

Summary of key commands for per neighbor TCP MSS

Table 1. Key commands
Command	Description
`show bgp neighbor`	Displays BGP neighbor details, including the configured MSS value.
`show tcp brief`	Lists active TCP connections and their states.
`show tcp pcb <pcb-value>`	Provides detailed TCP connection information for a specific PCB.

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TCP maximum segment size

Overview

Key attributes of MSS

Per neighbor TCP MSS

Key attributes of per neighbor TCP MSS

Configure per neighbor TCP MSS

Before you begin

Procedure

Example:

Example:

Example:

Example:

Example:

Example:

Disable the per neighbor TCP MSS

Before you begin

Procedure

Example:

Example:

Example:

Example:

Summary of key commands for per neighbor TCP MSS

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