Switch and Hub: what's the difference?

[Enrico Venturi]

Hello,
what's the difference between a Fiber Channel switch and a FC Hub?

thanks
Enrico

[Eugeny Brychkov · Accepted solution]

Enrico,
using FC hub you connect all devices to FC-AL, fibre channel arbitrated loop. One of devices is elected as masted and mastering this loop. Packets are passing through all the FC nodes - like in FDDI loops with hubs. So as more hosts/devices you add as much delays caused by devices retransmitting packets;
using switch you form SAN - storage area network. When you connect device to switch port, then there's a point-to-point (or 2 node loop, called fabric connection) automatically formed. All packets are routed by switch, so there're no useless packets sent to the device and device receives only packets sent to it. This is like LAN using routers/switches.
Theoretically, using switch and full duplex data throughtput can be 200MB/s for 1GBit/s connection.
In addition, it's recommended to connect tape devices to fabric switch, not to arbitrated loop. It is because when loop is reformed (for example, after adding or removing host(s)) then tape drive can receive reset request and backup job will fail/be aborted. In switch topology, when there're only 2 nodes in the dedicated loop (fabric connection), no broadcast 'reset' events are sent (if not sent directly to this switch port or storage device)
Eugeny