Switching

Hello all, until now I alays thought that preprovisioned is the much better and best practice mode to confgure VC and I used it for all new install, as it gives you better stability and predicatbility.
However after some testing, one problem with preprovisioned is that if both Master and Backup  members fail, the whole chassis seems to get irresponsible, because all other member are given "0" priority. I aggree that it is not common fail, but can  happen in case of power failure, especially with single PCU switches like EX3300.

> show virtual-chassis

Preprovisioned Virtual Chassis
Virtual Chassis ID: xxx
Virtual Chassis Mode: Enabled
                                           Mstr           Mixed Neighbor List
Member ID  Status   Serial No    Model     prio  Role      Mode ID  Interface
0 (FPC 0)  Prsnt    xxxxxxxx    ex3300-48p 129  Master*     NA  1  vcp-255/1/2
                                                               3  vcp-255/1/3
1 (FPC 1)  Prsnt    xxxxxxxx    ex3300-48p   0  Linecard    NA  2  vcp-255/1/2
                                                               0  vcp-255/1/3
2 (FPC 2)  Prsnt    xxxxxxxx    ex3300-48p   0  Linecard    NA  3  vcp-255/1/2
                                                               1  vcp-255/1/3
3 (FPC 3)  Prsnt    xxxxxxxx    ex3300-48p 129  Backup      NA  0  vcp-255/1/2
                                                                 2  vcp-255/1/3

While in a non-provisioned mode new Master will be elected if both Master and backup are lost and I presume the chassis should work with the remaining memebers:

> show virtual-chassis

Virtual Chassis ID: xxx
Virtual Chassis Mode: Enabled
                                           Mstr           Mixed Neighbor List
Member ID  Status   Serial No    Model     prio  Role      Mode ID  Interface
0 (FPC 0)  Prsnt    xxxxxxxxx    ex3300-48p 255  Backup      NA  1  vcp-255/1/2
                                                                 4  vcp-255/1/3
1 (FPC 1)  Prsnt    xxxxxxxxx    ex3300-48p 128  Linecard    NA  2  vcp-255/1/2
                                                                 0  vcp-255/1/3
2 (FPC 2)  Prsnt    xxxxxxxxx    ex3300-48p 128  Linecard    NA  3  vcp-255/1/2
                                                                 1  vcp-255/1/3
3 (FPC 3)  Prsnt    xxxxxxxxx    ex3300-48p 255  Master*     NA  4  vcp-255/1/2
                                                                 2  vcp-255/1/3
4 (FPC 4)  Prsnt    xxxxxxxxx    ex3300-48p 128  Linecard    NA  0  vcp-255/1/2
                                                                 3  vcp-255/1/3

Now I am bit uncertain of which method to use for future installs. What are you thoughts, which VC method do you prefer?

If you are sure that master and backup cannot go down in the same time while the other FPCs are up(basically ending in a headless VC), then you should go with preprovisioned mode. In this way you can control better what devices assumes the RE roles, what FPC numbers are allocated to the switches.

Otherwise, there is no drawback using a non-provisioned mode.

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If this worked for you please flag my post as an "Accepted Solution" so others can benefit. A kudo would be cool if you think I earned it.

Yes, you're right that in the preprovisioned mode, when all of the switches designated to routing engine role fail, the rest of VC is irrespnsible. In such case there is no Control Plane in the stack, even local switching does not work.

Preprovisioned mode may be useful in mixed VC configurations, when you want only certain switches to become RE. This mode may be also useful  when you are using advanced features, which  requires additional licences (it's enough to install such licences only on master and backup RE).

Personally ,I prefer dynamic (non provisioned) mode. You can achieve the above by configuring proper member's mastership priority.