Setting NIC's MTU permanently (viz. reboot consistent)

[Ralph Grothe]

Hello fellow admins,

encryption gateways require frames to have a maximum transfer unit size of some 1340 Bytes (probably because they need to wrap their header around the frames)

Although it was as easy as one single "lanadmin -M ..." command to set the MTU, I now wonder if I do have to provide my own init-oneliner "script" since I cannot find a config file where this is set "reboot-proof".

# lanadmin -m 0
MTU Size = 1340

# netstat -in|awk '{print$1,$2}'
Name Mtu
lan0 1340
lo0 4136

Because lan0 is using the btlan driver

# ioscan -kfdbtlan|awk 'NR<4'
Class I H/W Path Driver S/W State H/W Type Description
========================================================================
lan 0 0/0/0/0 btlan CLAIMED INTERFACE HP PCI 10/100Base-TX Core

it seemed obvious to look for a permanent config setting here

# grep -E '^[^#]' /etc/rc.config.d/hpbtlanconf
HP_BTLAN_INTERFACE_NAME[0]=lan0
HP_BTLAN_STATION_ADDRESS[0]=
HP_BTLAN_SPEED[0]=100FD
HP_BTLAN_INIT_ARGS="HP_BTLAN_STATION_ADDRESS HP_BTLAN_SPEED"

However, because of this comment in the file

# grep HP_BTLAN_INIT_ARGS /etc/rc.config.d/hpbtlanconf
# The HP_BTLAN_INIT_ARGS are reserved by HP. They are NOT user changable.
HP_BTLAN_INIT_ARGS="HP_BTLAN_STATION_ADDRESS HP_BTLAN_SPEED"


I do not dare squeezing in my MTU setting there.

The only init scripts HP provide that contain anything related to mtu belong either to Gigabit or FDDI

# grep -il mtu /sbin/init.d/*
/sbin/init.d/hpgelan
/sbin/init.d/hpiether
/sbin/init.d/hpigelan
/sbin/init.d/hppcifddi

# grep -il mtu /etc/rc.config.d/*
/etc/rc.config.d/hpfddi4conf
/etc/rc.config.d/hpgelanconf
/etc/rc.config.d/hpietherconf
/etc/rc.config.d/hpigelanconf
/etc/rc.config.d/netconf

# grep -in mtu /etc/rc.config.d/netconf
83:# "-f", "-n" and "-p pmtu".

# sed -n 80,85p /etc/rc.config.d/netconf
#
# ROUTE_ARGS: Route command arguments and options. This variable
# may contain a combination of the following arguments:
# "-f", "-n" and "-p pmtu".
#
# For each additional route, add a set of variable assignments like the ones


No, the last match isn't the right place either.
Does this mean HP don't provide a default place for setting MTU, and that I will have to extend their /sbin/init.d/hpbtlan script?

Rgds.
Ralph

[Sanjay Kumar Suri]

Hello Ralph

The man lanadmin has following to offer:

Changes made to an interface's station address or mtu interactively with the lanadmin command will not be preserved between system reboots. A user must modify the initialization configuration files for this feature, either manually editing configuration files or through the SAM interface.

Can you check SAM?

sks

[Ralph Grothe]

Hello Sanjai,

I almost never use SAM.
That's why I forgot to look there.
But having done so now I can only see the MTU of 1340 that I already changed manually with the lanadmin command.
But I have the creeping suspicion that this setting won't survive the next reboot.
That was why I was asking where HP hid the damned config file for MTU, or whether the admin should really be forced to modify the hpbtlan init script (which I cannot believe).
OK, this is not a feat.
But I wanted to follow the default guidelines, not to confuse some colleague, or vendor support later.

[Robert-Jan Goossens · Accepted solution]

Hi Ralph,

http://www5.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000062910454

How to set the MTU for EISA FDDI to 1500 bytes permanently DocId: KBRC00009957


Hope this helps,
Robert-Jan

[Ralph Grothe]

Hi Robert,

so it looks as if non standard MTUs is a not so common requirement, and the knowledge base article suggests what I would have done anyway.
Nevermind, instead of creating an extra init script I will modify the hpbtlan script that ifconfig-s the NICs.
Thanks for pointing me to the Knowledge Kase (KB) article.
Your query of KB must have been better than mine, which didn't reveal this article.

[rick jones]

It is good that you have a way out of the problem of persistent MTU setttings, but I have to ask, doesn't the encryption gateway support Path MTU discovery?