{master:0}
labroot@jtac-ex4300-32f-r2039> show virtual-chassis vc-port 
Jul 31 22:55:09
fpc0:  <<<<<<<<<<<<<<<<<
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
1/0         Configured          5    Up           40000        1   vcp-255/1/0
1/1         Configured          5    Up           40000        1   vcp-255/1/1


{master:1}
labroot@jtac-ex4300-32f-r2038> show virtual-chassis vc-port  
Jul 31 22:54:52
fpc1:   <<<<<<<<<<<<<<<<<<<
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
1/0         Configured          5    Up           40000        0   vcp-255/1/0
1/1         Configured          5    Up           40000        0   vcp-255/1/1

1.In a pre-provisioned VC setup, when replacing a member switch in the VC, verify if the Serial Number of the new switch is modified in the VC configuration of the master switch/ all the member switches.

2.If the serial number is not modified with the correct one, the replaced switch will not join the VC.

3.In the below example, in a 2 member VC, FPC0 is replaced but its serial number is not modified in FPC 1 hence they were both masters acting as individual devices and did not join the VC as shown below:

{master:0}. <<<<<<<<<<<<<<<<<
labroot@jtac-ex4300-32f-r2039> show virtual-chassis vc-port 
Jul 31 22:55:09
fpc0:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
1/0         Configured          5    Up           40000        1   vcp-255/1/0
1/1         Configured          5    Up           40000        1   vcp-255/1/1


{master:1}. <<<<<<<<<<<<<<<<<<<
labroot@jtac-ex4300-32f-r2038> show virtual-chassis vc-port  
Jul 31 22:54:52
fpc1:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
1/0         Configured          5    Up           40000        0   vcp-255/1/0
1/1         Configured          5    Up           40000        0   vcp-255/1/1

4. A point to note here is, when the 2 member switches are physically connected together, the new serial number of the replaced switch (FPC0 here) is rightly detected by FPC1 but, since the serial number is not changed manually in the pre-provisioned VC config in FPC1, there is serial number mismatch and thereby not joining the VC.

{master:1}
labroot@jtac-ex4300-32f-r2038> show virtual-chassis 
Jul 31 22:54:36

Preprovisioned Virtual Chassis
Virtual Chassis ID: 56ae.00ed.0131
Virtual Chassis Mode: Enabled
                                                Mstr           Mixed Route Neighbor List
Member ID  Status   Serial No    Model          prio  Role      Mode  Mode ID  Interface
1 (FPC 1)  Prsnt    TW3721020158 ex4300-32f     129   Master*      N  VC
-          Unprvsnd TW3721020099 ex4300-32f     <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< FPC1 detected the correct S.No. of FPC0 when physically connected together.



{master:1}
labroot@jtac-ex4300-32f-r2038> show configuration virtual-chassis 
Jul 31 22:53:30
preprovisioned;
no-split-detection;
traceoptions {
    file VCtrace size 20m;
    flag all;
}
member 0 {
    role routing-engine;
    serial-number TW3721020098;  <<<<<<<<<<<<<<<<<<<<<<<<<<<<< But S.No. is incorrect in the configuration, hence not joining the VC.
}
member 1 {
    role routing-engine;
    serial-number TW3721020158;
}


5.Corrected the Serial Number of the new replacement device FPC0 in FPC1 which fixed the issue.