Instead of making a minimum amount of bandwidth available for specific traffic types, you might want to limit available bandwidth. Both traffic policing and traffic shaping tools can accomplish this objective. Collectively, these tools are called traffic conditioners.
Policing can be used in either the inbound or the outbound direction, and it typically discards packets that exceed the configured rate limit, which you can think of a speed limit for specific traffic types. Because policing drops packets, resulting retransmissions, it is recommended for higher-speed interfaces.
Shaping buffers (and therefore delays) traffic exceeding a configured rate. Therefore, shaping is recommended for slower-speed interfaces.
Because traffic shaping (and policing) can limit the speed of packets exiting a router, a question arises:"How do you send traffic out of an interface at a rate that is less than the physical clock rate of the interface?" For this to be possible, shaping and policing tolls so not transmit all the time. Specifically, they send a certain number of bits or bytes at line rate, and then they stop sending until a specific timing interval (for example one-eighth of a second) is reached. After the timing interval is reached, the interface again sends a specific amount of traffic at the line rate. It stops and waits for the next timing interval to occur.
This process continually repeats, allowing an interface to send an average bandwidth that might be below the physical speed of the interface. This average bandwidth is called the committed information rate (CIR). The number of bits (the unit of measure used with shaping tools) or bytes (the unit of measure used with policing tools) that is sent during a timing interval is called the committed burst (Bc). The timing interval is written as Tc.
For example, imagine that you have a physical line rate of 128Kbps, but the CIR is only 64Kbps. Also assume that there are eight timing intervals in a second (that is, Tc = 1/8 second = 125 ms), and during each of those timing intervals, 8000 bits (the committed burst parameter) are sent at the line rate. Therefore, over the period of a second, 8000 bits are sent (at the line rate) eight times, for a grand total of 64,000 bits per second, which is the CIR.
If all the Bc bits (or bytes) are not sent during a timing interval, there is an option to bank those bits and use them during a future timing interval. The parameter that allows this storing of unused potential bandwidth is called the excess burst (Be) parameter. The Be parameter in a shaping configuration specifies the maximum number of bits or bytes that can be sent in excess of the Bc during a timing interval, if those bits are indeed available. For those bits or bytes to be available, they must have gone unused during previous timing intervals.
Policing tools, however, use the Be parameter to specify the maximum number of bytes that can be sent during a timing interval. Therefore, in a policing configuration, if the Bc equals the Be, no excess bursting occurs. If excess bursting occurs, policing tools consider this excess traffic to be exceeding traffic. Policing tools consider traffic that conforms to (that is, does not exceed) a specified CIR to be conforming traffic.
The relationship between the Tc, Bc, and CIR is given with this formula: CIR - Bc/Tc. Alternatively, theformulacan be written as Tc = Bc/CIR. Therefore, if you want a smaller timing interval, you can configure a smaller Bc.
About The Author
Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.
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