When operating in SmartFabric mode, each switch pair runs a VLT interconnect (VLTi) between them. For the MX9116n FSE, QSFP28-DD port groups 11 and 12 (eth1/1/37-1/1/40) are used.
For the MX5108n, ports 9 and 10 are used. Port 10 operates at 40 GbE instead of 100 GbE because all VLTi links must run at the same speed.
VLT backup link
A pair of cables is used to provide redundancy for the primary VLTi link. A third redundancy mechanism, a VLT backup link, is automatically created when the SmartFabric is created. This link exchanges VLT heartbeat information between the two switches using the management network to avoid a split-brain scenario should the external VLTi links go down. Based on the node liveliness information, the VLT LAG/port is in up state in the primary VLT peer and in down state in the secondary VLT peer. When only the VLTi link fails, but the peer is alive, the secondary VLT peer shuts down the VLT ports. When the node in primary peer fails, the secondary becomes the primary peer.
To see the status of VLT backup link, run show vlt domain-id backup-link.
For example:
OS10# show vlt 255 backup-link
VLT Backup Link
------------------------
Destination : fde1:53ba:e9a0:de14:2204:fff:fe00:a267
Peer Heartbeat status : Up
Heartbeat interval : 30
Heartbeat timeout : 90
Destination VRF : default