Hello, my friend.
The Eth-Trunk of the MA5600T is one of the types of link detected by BFD.
BFD detects communications faults between forwarding engines, specifically the connectivity of a data protocol on a path between systems. The path can be a physical link, a logical link, or a tunnel.
BFD can be regarded as a service provided by the system.
Figure 1 shows the types of links detected by BFD.
Figure 1 Types of links detected by BFD

Thanks.