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How CoS Applies to Packet Flow Across a Network

CoS works by examining traffic entering at the edge of your network. The edge routing devices classify traffic into defined service groups to provide the special treatment of traffic across the network. For example, voice traffic can be sent across certain links, and data traffic can use other links. In addition, the data traffic streams can be serviced differently along the network path to ensure that higher-paying customers receive better service. As the traffic leaves the network at the far edge, you can reclassify the traffic.

To support CoS, you must configure each routing device in the network. Generally, each routing device examines the packets that enter it to determine their CoS settings. These settings then dictate which packets are first transmitted to the next downstream routing device. In addition, the routing devices at the edges of the network might be required to alter the CoS settings of the packets that enter the network from the customer or peer networks.

In Figure 1, Router A is receiving traffic from a customer network. As each packet enters, Router A examines the packet’s current CoS settings and classifies the traffic into one of the groupings defined by the Internet service provider (ISP). This definition allows Router A to prioritize its resources for servicing the traffic streams it is receiving. In addition, Router A might alter the CoS settings (forwarding class and loss priority) of the packets to better match the ISP’s traffic groups. When Router B receives the packets, it examines the CoS settings, determines the appropriate traffic group, and processes the packet according to those settings. It then transmits the packets to Router C, which performs the same actions. Router D also examines the packets and determines the appropriate group. Because Router D sits at the far end of the network, the ISP might decide once again to alter the CoS settings of the packets before Router D transmits them to the neighboring network.

Figure 1: Packet Flow Across the NetworkPacket Flow Across the Network