Error "Unable to change MTU size: errno = 22" when changing MTU size

[aena]

My system is an HP-UX 11.11 on a rp4440 box.

The network card model is:

mad426de:root>ioscan -fnkC lan
Class I H/W Path Driver S/W State H/W Type Description
===================================================================
lan 0 0/1/2/0 igelan CLAIMED INTERFACE HP A6825-60101 PCI 1000Base-T Adapter
lan 1 0/2/1/0 igelan CLAIMED INTERFACE HP A6825-60101 PCI 1000Base-T Adapter
lan 2 0/5/2/0 igelan CLAIMED INTERFACE HP A6825-60101 PCI 1000Base-T Adapter
lan 3 0/6/1/0 igelan CLAIMED INTERFACE HP A6825-60101 PCI 1000Base-T Adapter

The command lauched:

mad426de:root>lanadmin -M 1300 0
Old MTU Size = 1500
Unable to change MTU size: errno = 22

I need to change the MTU size cause the server is in a VLAN tunnelised to the enterprise network with Nortel Opteras and these ones seem to add headers to the packet that eventually exceed the 1500 limit, what causes that 1460 bytes packets sent by the server get rejected by the network.

The symptoms are that telnets or ftps to servers (both peers HP-UX 11.11) outside the VLAN that require more than aprox 1450 bytes of information cause the session to hang.

Using other peers (changing MTU only on one of the peers is enough) that allow to change MTU to 1300, the problem gets solved.

An incident has been upgraded to Nortel, but if MTU could be changed on HP-UX servers I could keep on working on the applications installed on the servers on this network.

[aena]

Some additional data I got: hpigelan driver only admits MTU size of 1500 or 9000. The point is whether MTU can be changed any other way.

I have seen that /dev/ip includes the following parameters related to MTU
- ip_ire_pathmtu_interval
- ip_pmtu_strategy
but it doesn't seem of help in this case.

[Steve Lewis]

You could also use the route command to change the MTU down particular routes. See man route for more details.
That may work - try it - but keep yourself logged in on the console when making network changes like this, because you could kick yourself off.