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Two reasons for "high" fabric bandwidth. First, Cisco quotes fabric bandwidth as full duplex. (E.g. two full duplex gig ports would have a four gig fabric.)
Second, earlier switch fabrics often didn't support all their ports total aggregate bandwidth. With evolution of hardware, today the often do. This is more about bragging rights than actual need as very few drive all switch ports at their full bandwidth capacities concurrently. (BTW, similar issue with new switches' PPS rating - i.e. they often will handle all ports at line-rate, concurrently.)
