fabric physical up ,but Fabric link status: Down . why

show chassis cluster interfaces  
Control link status: Up

Control interfaces: 
    Index   Interface        Status
    0       em0              Up    
    1       em1              Up    

Fabric link status: Down

Fabric interfaces: 
    Name    Child-interface    Status
                               (Physical/Monitored)
    fab0    ge-0/0/1           Up   / Down
    fab0   
    fab1    ge-4/0/1           Up   / Down
    fab1