FreeBSD 9.0: higher MTU & NIC bonding

Here’s is some information that took me a good while to gather.

With the igb driver in FreeBSD, the mbuf cluster size needed is a mathematical formula involving the number of CPUs & the desired MTU. Unfortunately, it is currently hard set. On enterprise machines with many cores and higher MTUs, it is quite easy to reach this set limit. It will express itself with the following error message after an ifconfig:

igb0: Could not setup receive structures

This limit can be overridden with the following in /etc/sysctl.conf


These are the value that worked for 16 cores & an MTU of 9000.

While we’re at it, it took me a while to nail the exact syntax require for NIC bonding so here it is:


ifconfig_igb0=”mtu 9000 UP”
ifconfig_igb1=”mtu 9000 UP”
ifconfig_lagg0=”laggproto failover laggport igb0 laggport igb1 netmask″

As far as I can tell, capitalization matters…