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Whats so difficult about ISSU

The major vendors have figured it out for the most part by moving to stateful synchronization between control plane modules and implementing non-stop routing.  

ALU has supported ISSU on minor releases for many years and just added support for major releases. 

The Cisco Nexus ISSU works well, I've done an upgrade on a 5K switch and it was completely hitless. 

Juniper and Cisco with the 9K have gone through some hurdles but ISSU is actually usable now if the software versions support it.  

The main remaining hurdle is updating microcode on linecards, they still need to be rebooted after an upgrade.  


On Nov 8, 2012, at 6:22 PM, Kasper Adel <karim.adel at gmail.com> wrote:

> Hello,
> We've been hearing about ISSU for so many years and i didnt hear that any
> vendor was able to achieve it yet.
> What is the technical reason behind that?
> If i understand correctly, the way it will be done would be simply to have
> extra ASICs/HW to be able to build dual circuits accessing the same memory,
> and gracefully switch from one to another. Is that right?
> Thanks,
> Kim