When and Where to Use Shaping and Policing

Most implementations of shaping and policing occur at the edges between two different networks. For instance, consider Figure 5-1, which illustrates the two typical cases for shaping and policing. The figure shows PB Tents Enterprise network, with a Frame Relay service, and Internet connectivity using ISP1.

Figure 5-1 Connection to an ISP, and to a Frame Relay Network

PB Tents Enterprise Network

Figure 5-1 Connection to an ISP, and to a Frame Relay Network

PB Tents Enterprise Network

In this case, PB Tents has three separate boundaries between different networks. Link1, between R1 and the Frame Relay network switch labeled FRS1, is the first boundary. The second boundary is between the switch FRS2 and R2. Finally, a boundary exists between R3 and ISP-R1, over Link3.

For each boundary, the legal documents that detail the agreement between PB Tents and the Frame Relay service provider, and the documents that detail the agreement with ISP1, include something called a traffic contract. The need for the contract makes the most sense in the context of Frame Relay, but it also applies to the Internet connection. For instance, R2 uses a T/1 access link into the Frame Relay network. However, the virtual circuit (VC) between R2 and R1 may only include a committed information rate (CIR) of 64 kbps. Similarly, R1 has a 128-kbps access link, and the CIR of the VC to R2 still has a CIR of 64 kbps. When R1 sends a packet, the packet is transmitted at 128 kbps—that's the only speed that physically works! Likewise, R2 must send at 1.5 Mbps. However, the traffic contract may just state that the VC from R1 to R2 allows only 64 kbps in each direction.

Similarly, PB Tents and ISP1 may agree to install a link that is faster than PB Tents actually needs right now, expecting that PB Tent's Internet traffic loads will grow. When PB Tents needs more capacity to the Internet, each party can just agree to a new traffic contract, and PB Tents will pay ISP1 more money. For instance, metro Ethernet/Fast Ethernet/Gigabit Ethernet services have become more common; most sites do not really need 100 Mbps of bandwidth to the Internet. If PB Tents connected to ISP1 using a Fast Ethernet connection, but the traffic contract stated that PB Tents gets only 2 Mbps of service, however, the same mismatch between physical capability and legal ability, as seen with Frame Relay, occurs on the Internet connection.

In short, policing and shaping can play a role in cases where a router can send more traffic than the traffic contract allows. Shaping just slows the rate of sending packets so that the traffic contract is not exceeded. Policing discards some packets so that the traffic contract is not exceeded.

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