Redirected as this is a misc@ question not a tech@ question. On Thu, 2007-08-02 at 11:48 +0930, Hugo van Niekerk wrote: > Hey Everybody > > Im running OpenBSD 3.9. At startup during vlan initialization I get > an error that the vlan initiated with a nonstandard mtu of 1946 > (parent pcn1). Of course I understand that a vlan header has to be > sent and that provision has to be made for this. Ive done a lot of > searching on the web around this and everything is pointing to the > fact that the network driver does not support / has not been set for > a larger mtu. This is where I get stuck. There is a component > SIOCIFMTU that reports that the mtu value I use is incorrect for a > specific interface when I try to increase the mtu from the command > line using ifconfig. This component, according to my research sets > the mtu value automatically at startup based on the information that > is fed to it by the network card driver. Anything above what is set > at startup would be invalid. Can anybody please give me some advise > as to where I can more resources that I can research to resolve this > problem or point me in the right direction. I would really > appreciated this! Also, I've been looking for a guide or good book / > resource around configuring vlans on OpenBSD, I thought maybe I made > a mistake in the hostname.if and pf.conf files. Again any assistance > here will be greatly appreciated. > > regards > > Hugo >
If your interface is initializing with a MTU of 1496 then the card that you are using is not capable of supporting a MTU over 1500 which is required by the 802.1q spec to do VLAN tagging. A full size ethernet frame with the 802.1q encapsulation is actually 1504 bytes. This is not a configuration issue, unless you have specified a smaller packet size. There is no configuration needed, if the card is capable of doing vlan trunking with the full packet size, it will otherwise it will automatically decrease the MTU to allow for the 4 byte header that will be added. Unfortunately not all the ethernet cards/drivers are able to support this over-sized packet. To check which drivers are capable of supporting it you can grep for "IFCAP_VLAN_MTU" in the /usr/src/sys/dev/pci/if_<driver>.c files. I personally have had the best luck with the fxp, sis, and em drivers when I am doing vlan trunking. Tim Donahue
