Switch sees “wrong” MAC address on F5 interface (Confusion!)

Doing some troubleshooting on an F5 LTM it quickly became very confusing when the MAC address learned by the switch didn’t match up to the MAC address in the F5’s interface list (or ifconfig -a output, which matched the GUI).

After shutting down interfaces to verify exactly which interface went where, it took a while before we discovered that the F5 uses the MAC address assigned to the (in our case untagged) VLAN on that port.

ie: Network > VLANS : VLAN LIST > [VLAN NAME]

Select “Advanced” in the Configuration dropdown and the MAC address field will appear.

Hopefully this will save someone else from the confusion and/or headaches that this kind of thing causes.