The fab interface is a physical connection between two nodes of a chassis cluster that is used to forward traffic and synchronize session state between the nodes. The fab interface can be any pair of Ethernet interfaces on the same LAN, but they must be the same media type. You need to specify the physical interfaces to be used for the fab link in the configuration, as the system does not determine them automatically. The Junos OS supports only one fab link per node, and it does not support traditional interface features such as IP addressing, routing protocols, or firewall filters. The fab interface is assigned an internally derived IP address by the system for packet transmission. Thefab link also does not support fragmentation, so the MTU size of the fab interface must be equal to or greater than the MTU size of the largest interface in the cluster. References:

Chassis Cluster Fabric Interfaces

HA Chassis cluster, difference between Swfab and Fab