[c-nsp] MTU and PMTUD

[Nick Hilliard]

Marcin Kurek via cisco-nsp wrote on 08/12/2022 09:25:
> Interesting, but why would 'sh run' or 'write' raise an interrupt?
> Isn't this a branch in code that handles the CLI?

this was monolithic IOS running on older platforms. On a non-preemptive 
multitasking operating system like this, interrupts will interrupt. So 
once you have an interrupt, you need to wait until the interrupt handler 
is finished what it's doing before the kernel can continue with anything 
else.  If you have interrupt-driven flash i/o writes, with a CPU based 
forwarding plane -  which is what you had on everything from AGS to 7200 
- then packet forwarding is going to be impacted.

The effect was very obvious if you issued a flash write on an older 
device (e.g. C2500), while pinging the device.  The ping response times 
would increase up to 200ms.  Obviously packet forwarding is handled with 
much higher priority than ping, but it will be affected all the same.

Nick