Is jumbo ethernet MTU possible with Hamachi Gigabit ethernet driver?

I'm trying to get some Gigabit ethernet cards that use the Packet Engines Hamachi GNIC-II chip to use a large mtu to attempt to get a throughput of close to the 1Gb rating of the card. This is on a Compact PCI Alpha system. I'm trying to use an MTU in the 8000 to 9000 range and so far have not been able to get these MTUs to work.

I have changed the PKT_BUF_SZ and MAX_FRAME_SIZE constants in hamachi.c and ETH_DATA_LEN and ETH_FRAME_LEN in if_ether.h. I can use ifconfig to change the MTU above 1500 on one side of a connection but as soon as I raise the MTU on both sides to greater than 1500 the link dies. I can change the MTU with ifconfig back to 1500 and the link will resume operation. We are currently somewhat married to the 2.2.14 kernel.

I read that some ethernet drivers will support jumbo MTUs. There appears to be something in the hamachi driver or the kernel that I've missed. Perhaps this only works with a later version kernel or the hamachi driver needs more changes? Any help would be appreciated.

John Weidman
[email protected]

The hardware also must support jumbo MTUs. Hamachi limit is 1518 or 1522 (vlan) bytes, and the driver can't fix that.

		-- Pete