pfSense

In pfSense 2.2 (and I believe 2.1 and earlier as well), there is no way to properly set the MTU when assigning a LAGG port to an interface. In FreeBSD the LAGG MTU is inherited from the first physical port added to the LAGG group, but pfSense provides no mechanism for directly setting physical port MTUs, only abstract interface MTUs. There also doesn't seem to be any logic that takes the high level MTU of an interfaces and propagates it down to the underlying LAGG member ports. This effectively makes it impossible to use any value other than the default MTU (e.g. 1500) when using a LAGG port with an interface.

It seems like a simular issue might apply to setting the MTU of a VLAN interface, but since pfSense still allows one to create a separate dummy interface pointed directly at the physical port and set the MTU there, allowing the VLAN to inherit this MTU or set a lower value.

See https://forum.pfsense.org/index.php?topic=79180.0 for details and evidence.

There is also a (very) hacky workaround discussed at https://forum.pfsense.org/index.php?topic=50444.15.

It seems like there are at least a few ways to fix this:

+ Make the MTU settings a property of individual physical ports and vlans, instead of a property of the abstract interface. Then users can set this per-port/VLAN and have the LAGG ports (as well as any assigned interfaces) inherit the per-port/VLAN MTU value.
+ Allow LAGG member ports to be assigned to dummy interfaces such that the MTU can be set on them directly before they are joined to the LAGG group (seems prone to error).
+ Add logic to pfSense to properly take the abstract interface MTUs and push them down to the underlying LAGG member ports (this could get complicated, e.g. multiple VLANs atop a LAGG, each with different MTUs would require finding the largest VLAN MTU, and then making that the MTU of all the LAGG member ports, and thus the MTU of the LAGG port itself).

This may also realte to https://redmine.pfsense.org/issues/2504.

I'm not sure if the same issue exists for the MSS settings, but that may need to be considered as well.

It would be great to see this fixed to allow MTUs > 1500 in pfSense 2.2. It's one of those things that would be trivial to setup on a vanilla FreeBSD system but lacks a clean setup mechanism atop all the pfSense abstractions.