This article gives an explanation of the scenario: when creating virtual chassis with 2xEX4100-48MP and 1xEX4100-48P the trunk ID value is always -1, but when just on 2 EX4100 member VC is 5.
VC with 2xEX4100-48MP and 1xEX4100-48P
root@switch> show virtual-chassis vc-port
fpc0:
--------------------------------------------------------------------------
Interface Type Trunk Status Speed Neighbor
or ID (mbps) ID Interface
PIC / Port
1/0 Configured -1 Up 10000 2 vcp-255/1/1
1/1 Configured -1 Up 10000 1 vcp-255/1/0
1/3 Configured Absent
1/2 Configured Absent
fpc1:
1/0 Configured -1 Up 10000 0 vcp-255/1/1
1/1 Configured -1 Up 10000 2 vcp-255/1/0
fpc2:
1/0 Configured -1 Up 10000 1 vcp-255/1/1
1/1 Configured -1 Up 10000 0 vcp-255/1/0
VC with just 2xEX4100-48MP
1/0 Configured 5 Up 10000 1 vcp-255/1/1
1/1 Configured 5 Up 10000 1 vcp-255/1/0
1/0 Configured 5 Up 10000 0 vcp-255/1/1
1/1 Configured 5 Up 10000 0 vcp-255/1/0
Referencing to the following documentation:
https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/command/show-virtual-chassis-vc-port.html
Trunk ID: a positive-number ID assigned to a link aggregation group (LAG) formed by the Virtual Chassis. The trunk ID value is –1 if no trunk is formed. A LAG between uplink VCPs requires that the link speed be the same on connected interfaces and that at least two VCPs on one member be connected to at least two VCPs on the other member in an EX4200 or EX4500 Virtual Chassis.
Dedicated VCP LAGs are assigned trunk IDs 1 and 2. Trunk IDs for LAGs formed with uplink VCPs therefore have values of 3 or greater.
The trunk ID value changes if the link-adjacency state between LAG members changes; trunk membership is then allocated or deallocated."
1. Why is -1 the expected value for the 3-member VC?
This occurs because the connections do not form a trunk between the three members, and there is no lag between them.
2. Why does the value change to 5 when one member is removed?
With only two members remaining, the two form a bundle to establish their connection through the two links changing the value to 5. This situation is not related to LAG connections with other devices. The need for more cabling with a third member causes the stack to not form with just two links therefore not established as a 5 value anymore.