[c-nsp] Acceptable port configurations for ASR 9902 (gripe)

[Nick Hilliard]

Drew Weaver wrote on 31/01/2024 14:12:
> I know it is not the same but if it can do
> 1x100GE,1x100GE,1x100GE,1x100GE it would sort of follow that it can
> do 1x100GE,1x100GE,1x100GE,10x10GE

tbh this will depend on the hardware and how the gearboxes are set up. 
For example if you had a single ASIC/NPU with 400G forwarding capacity 
split out into 2x200G, with two gearboxes available on the two 
southbound paths, that might give you one of: 4x100G (i.e. gearboxes 
bypassed and 4 separate 100G ports directly connected to the ASIC/NPU), 
or 2x100G + one gearbox activated / taking all the traffic. The gearbox 
traffic could then be split out into various combinations of lower speed 
ports. But because you now have a breakout south of the gearbox, the 
architecture no longer has the physical capability to provide native 
100G ports. Oops.

This isn't necessarily an explanation of what the ASR 9902 is actually 
doing - it's just an example of how gearbox implementations can lead to 
unexpected outcomes.

Nick