Technical Tip : Switch-interface MTU is modified to match the interface with lowest MTU when it is added as a member

- This behavior is by design as when adding a member into bridge (system.switch-interface).

Switch-interface (bridge) MTU will be updated to the lowest MTU among members.

- For example:

Software switch name 'LAN' is configured with member interface 'lan3'.

In this scenario, Both (lan3 and LAN) have default MTU of 1500.

# config system switch-interface
    edit "LAN"
        set vdom "root"
        set member "lan3"
    next
end

lan3 Link encap:Ethernet HWaddr 04:D5:90:EE:82:31
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0 Bytes) TX bytes:0 (0 Bytes)

LAN Link encap:Ethernet HWaddr 04:D5:90:EE:82:31
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:2 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0 Bytes) TX bytes:180 (180 Bytes)

However once 'lan2' interface with MTU of 1400 is added as a member, Switch-interface modifies its MTU to match the MTU of lan2 as it has the lowest among the two member interfaces.

FGT40F-4 # show system switch-interface
# config system switch-interface
    edit "LAN"
        set vdom "root"
        set member "lan2" "lan3"
    next
end

lan2 Link encap:Ethernet HWaddr 04:D5:90:EE:82:30
UP BROADCAST MULTICAST MTU:1400 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0 Bytes) TX bytes:0 (0 Bytes)

lan3 Link encap:Ethernet HWaddr 04:D5:90:EE:82:31
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0 Bytes) TX bytes:0 (0 Bytes)

LAN Link encap:Ethernet HWaddr 04:D5:90:EE:82:30
UP BROADCAST MULTICAST MTU:1400 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:2 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0 Bytes) TX bytes:180 (180 Bytes)