Bandwidth of switch

[mmmartin823]

Hey all,

Very beginner type question here...I don't understand the concept of total bandwidth of a switch.  For instance an access switch with 48 Cooper ports is capable of "X" Gbps of bandwidth... How is this calculated and why is this important if you know you get a 1G on each port?

[Joseph W. Doherty · Accepted solution]

Okay, understand the hardware that actually transmits/receives frames on a port, externally, generally isn't the same hardware that transmits/receives frames between ports, internally. Switches have internal capacity limits, for bandwidth and/or frames per second, which do not always support all the switch's external ports at their full capacity.

For a 48 ports at gig, a switch would need an internal capacity of 96 Gbps (assuming duplex ports) and a packet per second capacity of (about) 72 Mpps (to support minimum size Ethernet packets - required rate decreases with increased packet size, for same bandwidth, and also varies based on media's frame size requirements [e.g. VLAN tagged frames are larger]).

[mmmartin823]

Thank you so much for this answer.  helps me a lot!  So as a follow up, I'm looking at the screenshot attached for the bandwidth specifications of a Catalyst 9300 switches.  The switching capacity for the CAT9300-24T shows 208 Gbps and the Switching Capacity with stacking 688Gbps.  Can you help me with the math on these bandwidth numbers?  Are they including maxing out the uplink ports on the optional network modules into these numbers?   Thanks in advance!

[Joseph W. Doherty · Accepted solution]

The CAT9300-24T, besides having 24 triple speed copper ports, supports a module port that can provide two 40 Gbps port. So, the maximum internal bandwidth needed, for external ports, would be ((24 + (40 * 2)) * 2 for duplex) 208 Gbps. StackWise-480 needs another 480 Gbps (already calculated for duplex), bringing the total to (208 + 480) 688 Gbps.

Oh, and for those bandwidths, multiple half the bandwidth by 1.488 Mpps (per gig) will provide the Mpps rates.