50% of 128 kbps = shaping rate of 64 kbps. With the example above we are sending traffic 50% of the time and pausing 50% of the time. Most Cisco routers have a Tc default value of 125 ms. In total there are 8 time intervals of 125 ms each. This first interval takes 125 ms (62.5 + 62.5 = 125 ms) and we call this interval the Tc (Time Interval). The first 62.5 ms we are sending traffic at 128 kbps and the second 62.5 ms we are pausing. The green line indicates when we send traffic and when we are pausing traffic. Horizontally you can see the time from 0 to 1000 milliseconds. On the vertical line you can see the physical bitrate of 128 kbps. Now you have a basic idea of what shaping is, let’s take a look at a shaping example so I can explain some terminology:Ībove we see an interface with a physical bitrate of 128 kbps that has been configured to shape to 64 kbps. This means we will send 75% of the time and pause 25% of the time (96 / 128 = 0.75).
50% of 128 kbps = an effective CIR of 64 kbps.Īnother example, let’s say we have the same 128 kbps link but the CIR rate is 96 kbps. If we want to achieve this we need to make sure that 50% of the time we are sending packets and 50% of the time we are pausing.
FORTINET DOWNLOAD CAP TRAFFIC SHAPER SERIAL
If we want to get a lower bitrate we will have to send some packets, pause for a moment, send some packets, pause for a moment…and so on.įor example let’s say we have a serial link with a bandwidth of 128 kbps. As network engineers we think we can do pretty much anything but it’s impossible to make an electrical or optical signal crawl slower through the cable just because we want to. Routers are only able to send bits at the physical clock rate. In short, we configure shaping when we want to use a “lower bitrate” than what the physical interface is capable of. We can use shaping to make sure everything will be sent (until its buffer is full). When you go from a high speed interface to a low speed interface you might get packet loss (tail drop) in your outgoing queue.
Shaping is a QoS (Quality of Service) technique that we can use to enforce lower bitrates than what the physical interface is capable of.
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