In terms of capacity of bandwith, what is the best mode? crossbar mode or bus mode?
The bus mode is shared at half duplex 32 Gbps between the linecards and the sup engine. All packets transitted on the bus is sent to all cards.
The crossbar provides a dual dedicated faric channels for fabric enable line cards to transit and receive data which is much more faster than shared bus. Sup 720 provides crossbar archecture.
in terms of performance crossbar mode is better it means usage of the switching fabric on the switch, with current sup720 this means connection(s) to the 720 Gbps switching fabric.
the bus means the 32 Gbps (half-duplex) shared bus.
This is used by legacy linecards and by some service modules like CSM.
to be noted in the same chassis different modules can be in different mode.
Hope to help