I just want to briefly mention the reality of marketing math when you're considering vendor data sheets. It is very common for vendors to double-count bps or pps capacity when you have full-duplex links. For instance, Cisco's Catalyst 6500 has a Supervisor 720. 720 is used because it is marketed as having 720Gbps of fabric capacity.
But... if you ask how Cisco calculates 720Gbps, the answer is:
- 80Gbps of fabric per slot times 9 slots in a chassis
The issue here is their fabric is only a 40Gbps full-duplex fabric... However, Cisco counts both ingress and egress in the 720G number, even though it doesn't really make sense to count like that. The take-away is you sometimes need to be careful and inspect how you can apply the numbers in vendor literature to reality.
Almost every vendor twists marketing numbers like this, and I only pick the Cat6500 because I'm very familiar with the platform. This is not a condemnation of Cisco or the Cat6500 (which I actually have quite a passion for).
What is the exact meaning of each figure? What is the difference between them?
- Number of bits of data per second that can be processed without dropping data. bps is almost always measured using 1500 byte (or potentially larger) ethernet payloads.
- bps is frequently used when measuring the capacity of components which interconnect multiple linecards or ports within a chassis (like a switch fabric). Occasionally, a central processing engine might have a bps limitation...
- Number of packets of data per second that can be processed before dropping data; pps is always measured using the smallest packet sizes possible.
- pps is frequently used when measuring components that look inside a packet header (for an IP address, mac-address, DSCP value, etc...). For example, the capacity of route and switch processors are measured in pps.
When should I focus on each value for switch assessment?
There is a time and place for this kind of analysis, but most people only use a tiny fraction of their switch pps / bps capacity, unless it's a top of rack switch in a busy data center, or a core switch for a mid to large service provider POP.
Even so, vendor sales staff might not be interested in highlighting the product limitations, or may not understand the limits well themselves. Also, the packet-per-second numbers often change depending on the features, or combination of features that you turn on... there really is no substitute for coming up with a few good test cases, and testing the performance of the box with the combination of features you think you realistically need.
Due to variances and the games people can play with numbers on data sheets, the most important thing you can ask the vendor is "show me how you calculated the bps and pps numbers for this component".
That said, hardware and software features are as important, or more important to consider than drag-race bps / pps numbers... I'm including a small sample of items you might want to look at... this is very subjective...