CoS Across the Network
CoS works by examining traffic entering at the edge of your network. The edge devices classify traffic into defined service groups, which allow for 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 device in the network. Generally, each 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 device. In addition, the devices at the edges of your network might be required to alter the CoS settings of the packets transmitting to the neighboring network.
Figure 1 shows an example of CoS operating across an Internet Service Provider (ISP) network.
In the ISP network shown in Figure 1, Device A is receiving traffic from your network. As each packet enters, Device A examines the packet’s current CoS settings and classifies the traffic into one of the groupings defined by the ISP. This definition allows Device A to prioritize its resources for servicing the traffic streams it is receiving. In addition, Device A might alter the CoS settings (forwarding class and loss priority) of the packets to better match the ISP’s traffic groups. When Device B receives the packets, it examines the CoS settings, determines the appropriate traffic group, and processes the packet according to those settings. Device B then transmits the packets to Device C, which performs the same actions. Device D also examines the packets and determines the appropriate group. Because it sits at the far end of the network, the ISP might decide once again to alter the CoS settings of the packets before Device D transmits them to the neighboring network.