Netgate 7100 internal switch MTU question

clopedia89

I have 3 WANs on the 7100's internal switch that require reduced MTU, and while trying to set that, it told me I can't set it higher than the parent interface's MTU. This obviously makes sense, but it got me thinking: why is the parent's MTU so low?

I have my internal switch setup as 8 isolated ports, each on it's own VLAN and assigned as a separate interface.
I opened a shell at the console and ran ifconfig to discover the following:

All 8 VLANs for the internal switch: MTU 1430
lagg0 MTU: 1430
ix2/ix3 MTU: 1430

I have never explicitly set an MTU anywhere except a single one of those VLANs (for a very finicky cellular link). I would have expected lagg0, ix2, and ix3 to all be 1500, allowing me to freely pick various MTU sizes for the VLANs.

Can anyone explain what's going on here? Is it normal for the link to the internal switch on the 7100 to have a reduced MTU, or did I unknowingly trigger this at some point in the past?

clopedia89

This post is deleted!

clopedia89

Actually, I need to correct myself:
My settings are as follows:

switchport |  interface | webGUI setting | ifconfig output
==========================================================
         1 | lagg0.4001 |     [blank]    |    MTU 1430
         2 | lagg0.4002 |     [blank]    |    MTU 1430
         3 |            |                |
         4 | lagg0.4004 |     [blank]    |    MTU 1430
         5 |            |                |
         6 | lagg0.4006 |    MTU 1492    |    MTU 1430
         7 | lagg0.4007 |     [blank]    |    MTU 1430
         8 | lagg0.4008 |    MTU 1430    |    MTU 1430
           |    lagg0   |                |    MTU 1430
           |     ix2    |                |    MTU 1430
           |     ix3    |                |    MTU 1430

A couple switchports are unused and thus I have not created a VLAN in pfSense
(reposted to correct a confusing mistake, sorry folks, it's been a long week and I'm tired)

stephenw10

It's probably the VLAN setting propergating to the parent NICs and then getting applied to all VLANs on it.

You can usually work around that by explicitly setting an MTU value on the parent interface first. But you may need to temporarily remove the lower MTU on the VLAN to do so.