This article goes over FPC issues when a chassis cluster is set up with the control link connected through a broadcast domain, instead of P2P.

• Non-P2P control link, with a L2 enviroment connecting the control ports of each node.
• The secondary node's FPC will never come up.
• No HW error logs or alarms.
• Nodes temporarily appear as Lost from time to time.

In chassis cluster mode, the secondary node's FPC connects to primary node's chassisd. Aka secondary FPC connects to Primary RE using the control link.

Meaning, that if there's anything that causes packet loss between the control ports, this scenario may be seen. For example, there may be some encapsulation that causes the control packets to be dropped due to MTU issues, in which case increasing the MTU size will fix the issue and bring the FPC up.