How do I set a network bridge to have an MTU of 9000?

Solution 1:

Looks like the mtu option is not available when using the manual method (see interfaces(5)). So, here's what is supposed to work (incorporating the feedback from the comments):

auto lo
iface lo inet loopback

# Set up interfaces manually, avoiding conflicts with, e.g., network manager
iface eth0 inet manual
   # nothing here

iface eth1 inet manual
   # nothing here

# Bridge setup
auto br0
iface br0 inet static
   bridge_ports eth0 eth1
   address 192.168.88.2
   ...
   post-up ifconfig eth0 mtu 9000 && ifconfig eth1 mtu 9000

Using the up (or in this case post-up) option we can specify our own command to run during (of after) the time the interface is brought up.