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Changing MTU on TCPIP network

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Kitti Thanapuasuwan
Occasional Advisor

Changing MTU on TCPIP network

I try to change MTU size from 1500 (standard) to 1498 by creating the .com file. The .com file is added into SYSTARTUP_VMS.COM. After the system reboot, it didn't change the MTU size.
The .com file look like below (only 2 lines);
ifconfig we0 ipmtu 1498
Did I do something wrong?

John Gillings
Honored Contributor

Re: Changing MTU on TCPIP network


Was this done before or after starting TCPIP? I think to make it work, you would need to execute the command BEFORE starting the interface (or stopping and starting the interface after setting the value)

However, the question arises - why? With current versions of TCPIP, you're much better off letting Path MTU Discovery work it out for you.
A crucible of informative mistakes
Honored Contributor

Re: Changing MTU on TCPIP network

you should put your command in the file tcpip$systartup.com, which (if it exists) is called at the beginning of tcpip$startup.com.

This file is the right one for modifications of Tcpip before starting, for example

$ @sys$startup:tcpip$define_commmands
$ sysconfig -r inet tcp_msl=32

(this is only an example !)

Ian Miller.
Honored Contributor

Re: Changing MTU on TCPIP network

tcpip$systartup.com is called at the end of tcpip$startup.com. It is the usual place for setting tcpip paramters.
Purely Personal Opinion
Kitti Thanapuasuwan
Occasional Advisor

Re: Changing MTU on TCPIP network

Dear John,

Because of the router/juniper that I have to connect, right now support only MTU size 1498 or smaller. This for temp solution until upgrade the new version of that equipment.

Once I see the mtu didn't change, I can sign on SYSTEM account and call manually. It means I can change it after TCPIP started.

Dear Gerard,
I should use command
sysconfig -r inet tcp_msl=32
instead of
ifconfig we0 ipmtu 1498
Please advise

Martin P.J. Zinser
Honored Contributor

Re: Changing MTU on TCPIP network

Hello Kitti,

first I seond the suggestion to create

sys$startup:tcpip$systartup.com and place your
commands in there. This is the supported way of changing IP settings and ensures the commands are executed at the appropriate time.

Second, your command "ifconfig we0 ipmtu 1498"
is correct, Gerard was just giving an example of what he has in his startup file.

Greetings, Martin