How to modify the MTU size on a NetScaller Load Balancer

This document describes how to change the Maximum Transmission Unit (MTU) size(s) like 1300 or 9k.

Requirements Background

MTU of 1514 = TCP MSS of 1460 (Difference = 54 bytes)
MTU of 1300 = TCP MSS of 1246
This applies to TCP MSS, and does not apply to MTU for all other types of Ethernet frames.

Procedure to modify the MTU
1. Execute the following command from the shell:

For 1300b use

nsapimgr -ys ns_max_mss=1300

For 9K use

nsapimgr -ys ns_max_mss=8946

(1460 is the default value for the MSS used by NetScaler.)

2. Save this command in rc.netscaler.

A better approach would be to allow the NetScaler to auto discover the MTU size by using the PMTUD command. At the shell prompt enter:

> enable ns mode pmtud
Path MTU Discovery enabled.
Done

The NetScaler uses Internet Control Message Protocol (ICMP) messages to auto discover the best MTU size.

Refer to the following links for background information:
http://www.webopedia.com/TERM/M/MSS.html

http://www.webopedia.com/TERM/M/MTU.html