A switching fabric is like a switched architecture inside the switch. Some switches have only a bus architecture. Think of it like this:
Bus is like old 10Base2. All ports see all packets. Access to the bus is arbitrated.
Switching fabric is like a little ethernet switch inside the box-each port can xmt/rcv at the same time.
Forwarding BW refers to how much bandwidth the whole switch offers (most of the time represented by full duplex numbers). E.g. if you had a 2950G-24 port switch (which has 24 10/100 ports and 2 Gig ports) you'd have a forwarding bandwidth of 24 X 100Mbps X 2 (full duplex) + 4 Gbps (2 gig ports full duplex) = 8.8 Gbps as seen here:
http://www.cisco.com/en/US/products/hw/switches/ps628/products_data_sheet09186a00801cfb64.html
Forwarding rate is the rate (usually expressed in 64byte packets per second) at which the switch can move data across the fabric. A normal fast ethernet port can handle about 148,800 packets in a sec (using a 64 byte frame and counting for packet overhead). Therefore a 24 port 100Mpbs switch would need to be able to move 3571200 packets in a second to ensure no packets are dropped.
Let's look at the 2950 24 port box.
According to the above URL, it has a 13.6 Gbps switching fabric. This is the full duplex bandwidth of the path inside the switch the ethernet ports use to communicate with each other internally.
The forwarding bandwidth is 8.8 Gbps. This is just another way of saying the switch has 24 100 Mbps full duplex ports and 2 Gig full duplex ports. If this number is less than the switching fabric bandwidth, the switch is said to be "non-blocking".
The forwarding rate of the 2950 24 port (+2 Gig) is 6.6Mpps @ 64byte packet size. This isn't a very real world scenario as most packets are larger than 64 bytes. This value simply means the switch is capable of moving 6.6M 64-byte packets across the fabric in one second. If we look at the 2950 24 port port + 2 Gig port switch:
148,800 * 24 = 3571200 (this is for the 100 Mbp ports)
1,480,000 * 2 = 2960000 (this is for the 2 Gig ports)
add them up and we get: 6531200 packets per second which is pretty close to what Cisco lists on the 2950 data sheet. This means this switch would not drop any frames if all the ports were lit up for one second.
HTH
