How to set MTU parameter permanently

[Steven Lam_2]

Hi,
I would like to set the MTU parameter for HP-UX 10.20 OS permanently. I don't want to use the default one. Your hints are greatly appreciated.

Steve

[Steffi Jones_1]

Hi,

change the lanadmin manpage for more info.

Steffi Jones

[Ajitkumar Rane]

Well I guess you can use "lanadmin" just execute lanadmin and it will bring up the menu.It allows you to do changes to some of the parameters.

[Steven Lam_2]

Thanks for quick respond. However, I got the error 22 when I try to change the MTU parameter by using lanadmin command.

Steve

[rick jones]

what s the current MTU, and what MTU do you want to use? is there a particular problem you are trying to solve by altering the MTU?

most links have maximum MTUs. also, i'm not sure that altering the MTU of the link is supported by all drivers (especially under HP-UX 10.20) - you might make sure you have the latest rev of the driver and/or patches.

if that does not work, you could probably do something with setting PMTU's in the routes. It would probably mean some editing of the netconf file and perhaps some other scripting.

[Alexander M. Ermes]

Hi there.
Please check your netconf in /etc/rc.config.d
Check the line with the ROUTE_ARGS. It should then show like this :

ROUTE_ARGS[n]="-p 1492"

if the permanent mtu size should be 1492.
[n] means the number of the lan interface.

Rgds
Alexander M. Ermes

[rick jones]

while the numbers are indeed often the same, the subscripts in the netconf file do not specify the number of the interface - as in the N in lanN. so, it is best not to rely on that coincidence.

[Steven Lam_2]

Hi,
We are using the encripted tunneling (VPN) and add a big header to the packet. So, when the network is congested the link is broken. That's why we would like to set the MTU parameter to optimize the network. Even by setting the host route on the netconf in /etc/rc.config.d, it doesn't overwrite the default route either. Thanks.

Regards,

Steve

[rick jones]

if the VPN destinations are "remote" (not in the same network/subnet in the context of IP) you might be able to get the desired effect by disabling PathMTU discovery with nettune (10.20) or ndd (11.X). if PMTUD is off, and the destination is "remote" the machine has to use an MSS of 536 bytes for TCP connections.

however, it won't do anything for UDP and thus NFS, though for NFS you could fall-back on reducint the mount size (which should probably be done for a WAN NFS mount anyway...)