Cheatsheet for IPv4 MTU in ExtremeWare 7.x

# mtu cheatsheet
# ping -Ds <n>          vlan mtu        parent vlan     extreme         extreme
# max size              ifconfig mtu    ifocnfig mtu    conf ip-mtu     conf jumbo-frame size
# 1472                  1500            1500            1500(max)       jumbo off
# 9166                  9194            9194            9194(max)       9216
# 8950                  8978            8978            8978            9000
# 8954                  untagged        8982            8982            9000
# summary: ping -Ds             x - 28
#          ping ``bytes from''  x - 20
#          ifconfig/ipmtu       x
#          jumbo size untagged  x + 18
#          jumbo size tagged    x + 22
#
# freebsd caps the MTU of vlan interfaces to that of the parent, so the largest untagged 
# frame it's ever capable of sending is 4 bytes shorter than whatever the largest 
# tagged frame you can configure it to send.
#
# extreme, however, will let you configure ip-mtu up to 9194 even if you shrink 
# jumbo-frame size, so it's possible to misconfigure it.  but it does permit a correct 
# configuration that FreeBSD doesn't: you could have an untagged vlan and a tagged vlan 
# on the same port, and set ip-mtu for the tagged to <x> and ip-mtu for the untagged 
# to <x + 4>.  but, this is slightly silly, because jumbo-frame size is a switch-wide 
# configuration and not analagous to the mtu of parent interfaces on FreeBSD.
#
# take-home message:
ifconfig_em0="mtu 9194"

ifconfig_vlan12="vlan 12 vlandev em0 mtu 1500"
ifconfig_vlan13="vlan 13 vlandev em0"