The MX LAN port connecting to the switch should be a trunk port. The native vlan will typically be either the management vlan of the switch (if you haven't configured the management vlan in the switch configuration), an unused VLAN, or none. ... View more

Ok, they're both capable of 10Gbps. Sounds like it could be an issue with the fiber then. I would do a loop back test on both sides to rule out the switches and SFP's  ... View more

How can I verify if that was the case? The SFP+ port on the primary switch still shows as enabled: I've also tried multiple SFP ports on the secondary switch to no avail.  The primary switch is set as bridge priority 4096 in switch settings. All other switches would have default bridge priority.  ... View more