[j-nsp] ISSU offlined mpc - why?

[Chuck Anderson]

On Wed, Sep 01, 2021 at 09:44:08AM -0700, Mike via juniper-nsp wrote:
> "Unified ISSU is supported with Junos OS Release 17.4R1 for MX Series 
> routers with MPC-3D-16XGE-SFPP"
> 
>      my expectations were that the card would stay online and there 
> would be little to no operational impact, but this clearly wasn't the 
> case. I am just wondering if anyone can tell me why this might have been 
> the case and in the future what steps I would need to take to ensure 
> this both otherwise stays on the air during a future software update?

Eventually during the ISSU process, the line card software needs to be
upgraded.  During that part, each line card goes offline one at a
time.  If you have multiple line cards, design your network such that
redundant network paths are connected across different cards to
prevent a total outage when each line card is upgraded one-by-one.