Switching Bandwidth and Packet Forwarding Rate Explained
When you’re purchasing switches, you can find a lot of different numbers posted. They all have important meanings, and they all inform your purchase. If you aren't already deeply familiar with each metric, then it’s worth taking time to learn the essentials.
What Is Switching Bandwidth?
The simplest way to think of switching bandwidth is that it’s the total amount of data a switch can send and receive each second. Highly technical definitions might discuss the transmission of data through the switching plane or packets and connected devices. All of that is true, but it’s not necessary to complicate the concept.
In fact, you can calculate switching bandwidth very easily. Take the maximum speed for every port and add it together. Then, multiply that number by two, and we have our bandwidth.
Why do we multiply by two? The switching bandwidth totals the data capacity for both incoming and outgoing packets, so with duplexing, your bandwidth is double the speed you can get in either direction. Put more simply, data can flow in both directions, and bandwidth defines the total amount of data in both directions.
The point of all of this is that your bandwidth tells you how much data the switch can handle. When purchasing switches, you want a bandwidth that is high enough to support your devices and applications.
The last thing to note about bandwidth is that it can be a little tricky. Switches come in all shapes and varieties, and not all of them support full duplexing. If you want to be sure that a switch can fully duplex, then the reported bandwidth needs to be double the sum of all port capacity (as mentioned before). If those numbers don’t seem to add up, then the switch you’re looking at won’t be able to provide full duplexing when you max out all of the ports.
In short, it will have bottlenecks and run slowly when you use the switch to its full abilities.
What Is Packet Forwarding Rate?
While that covers switching bandwidth pretty well, another important metric is packet forwarding rate. As the name implies, this is a measure of how many packets a switch can forward in a second. In fact, this metric is measured in packets per second (pps).
One thing of note with forwarding rates is that they are not theoretical. This number is produced by direct measurements, and it will prove variable while the switch is in use. When comparing switch literature, you might see maximum packet forwarding rates or specific rates depending on the minimum packet size. These posted rates can help you when purchasing a switch, but it’s important to remember that the numbers you see in practice will vary.
That brings us to another important point. PPS doesn’t mention how big the packet is. In fact, forwarding rates aren’t universal, as your packet sizes can and will vary while using a switch. That means you can’t exactly tell how fast a forwarding rate is just by looking at the number of packets per second.
Fortunately, there’s a standardization you can use to make cleaner comparisons. When using Ethernet, there is a set minimum packet size of 64 bytes (this does not include the preamble or interframe gap). If you assume that all of the packets are 64 bytes, then your forwarding rates are giving you direct data rates.
Again, not all packets will be 64 bytes, but you can use this as a basis for comparison, and when you check your switch’s literature, it will probably report packet forwarding rates without specifying a packet size. In that case, the industry standard assumes Ethernet packet sizes, and you can assume that the reported numbers refer to rates using a 64-byte packet.
That’s a lot of talking that boils down to a simple concept. Assume your packets are 64 bytes unless you have a good reason to stop.
Comparing the Two
Considering all of this, what’s the point? Sure, these metrics can help with switch selection, but why are they paired together in this article?
The answer to that lies in the relationship between forwarding rates and bandwidth.
Technically speaking, these data rates can be designed independently from each other in a switch. In other words, it’s possible to have a switch with a forwarding rate that doesn’t match the bandwidth, in which case you can get performance problems.
In fact, there’s a bit of math you can do to ensure you won’t run into these problems. Before that, we have to unlearn something from the previous section. When you’re calculating minimum packet sizes (which we’re about to do), you stop assuming that the packets are 64 bytes. In fact, that’s the point of this calculation.
Ok. Start by getting your switching bandwidth. Add up the port speeds and multiply by two. Once you have that number, divide it by the packet forwarding rate. Typically, that rate is 1.488 Mpps for every 1 Gbps of bandwidth. Now that you have these numbers, divide the bandwidth by your total forwarding rate, and that will give you the minimum packet size.
This number is important. If your switch tries to send packets smaller than what you calculated, then the forwarding rate won’t be fast enough to run the switch at its full capacity. You can’t speed up the forwarding rate, so if the packet sizes get small enough, you’re simply not sending as much information every second. It’s an important comparison to keep in mind when purchasing and also when configuring switches.
Buying Switches
The last application of this knowledge comes back to switch purchasing (which has been mentioned many times already). Let’s recap the ideas.
First, you want to get a switch with enough bandwidth for your purpose. As an example, if you want to connect a bunch of cameras to the switch, you need enough bandwidth for the total speed concerns of every camera added together.
From there, you can check your bandwidth against your forwarding rate. As long as the forwarding rate is sufficient, then you won’t bottleneck your own switch. And when in doubt, assume you’re working with Ethernet packets which bottom out at 64 bytes. Put an easier way, you need 1.488 Mpps for every 1 Gbps of bandwidth. Keep to those numbers, and you’ll be in good shape.
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