On Thu, Jun 04, 2026 at 10:52:10AM +0200, Thomas Pries wrote: > hi, > > I setup: > > - a LACP-trunk (aggr0) on two interfaces (ix0, ix1) > > - two vlan interfaces over this trunk (vlan1002, vlan689) > > - and a carp interface on each vlan interface > > -- carp1 on vlan1002 > > -- carp2 on vlan689 > > I checked the hardware features: > > ifconfig ix0/ix1?? hwfeatures ...??hardmtu 9198 > > ifconfig aggr0?? hwfeatures ...??hardmtu 9198 > > ifconfig vlan1002?? hwfeatures ...??hardmtu 9198 > > ifconfig carp1?? hwfeatures?? ...??hardmtu 1500 > > Does it make sense to increase the mtu on the ix* interfaces when the mtu of > then carp interfaces can't be set to a value greater then 1500?
OpenBSD is different to most other systems in how it handles packet sizes and MTU on interfaces. OpenBSD generally tries to set up network interfaces so they will operate with their maximum supported packet size at all times. This means that they're set up to be able to receive large packets at any time. This in turn means that the MTU on an interface is a purely software setting used by the IP stack to decided what size packets can be sent to the network. It is not used to imply the maximum receive unit (MRU) size on an interface which is often used in other systems to decided what packet size the hardware should be configured to handle. In practice this means that you can set the MTU on aggr and vlan interfaces separately to the MTU on the parent interfaces. If your ix interfaces are used by aggr, then it doesn't matter what the MTU on the ix interfaces are because aggr has taken the ix interfaces away from the IP stack. It's the MTU on the aggr and vlan interfaces that will be used by the IP stack instead. To a large degree it also doesn't matter what the MTU on the carp interfaces is either. Except for some very nich configurations, packets are only ever received by carp interfaces, and OpenBSD will accept packets of any size supported by the underlying hardware, regardless of the MTU. In your set up it's the MTU of the vlan interfaces that matters, but only to the IP stack. Everything underneath it is already set up for you to be able to increase the MTU on the vlan interfaces. > > Can other (future) vlans without carp benefit from a larger mtu on ix*? > > When I try to set the mtu: > > ifconfig ix0 down > > ifconfig ix0 mtu 9180 > > ifconfig: SIOCSIFMTU: Device busy > > > Thanks in advance. > > > Kind regards > > Thomas > > > > > > > > > Mit freundlichen Gr????en > Thomas Pries > -- > > > Thomas Pries > Mitarbeiter Rechenzentrum > ___________________________________ > Hochschule f??r Grafik und Buchkunst Leipzig > Academy of Fine Arts > W??chterstr. 11 | 04107 Leipzig > Postfach 10 08 05 | 04008 Leipzig > Tel.: +49 341 2135-106 > https://www.hgb-leipzig.de >
