Packet Tagging Overview

You can use the traffic-class rule in policies to tag a packet flow so that the QoS application can provide traffic-class queuing. Policies can perform both in-band and out-of-band packet tagging:

• Policies perform in-band tagging by using their respective mark rule to modify a packet header field. For example, IP policies use the mark rule to modify an IP packet heard ToS field, and Frame Relay policies use the mark-de rule to modify the DE bit.
• Policies perform out-of-band tagging by using the traffic class or color rule. Explicit packet coloring lets you configure prioritized packet flows without having to configure a rate-limit profile. The router uses the color to queue packets for egress queue threshold dropping as described in Creating Rate-Limit Profiles.

For example, an Internet service provider (ISP) provides a Broadband Remote Access Server (B-RAS) service that has both video and data components, and the ISP wants to guarantee that the video traffic gets priority treatment relative to the data traffic. The ISP’s users have a 1.5 Mbps virtual circuit (VC) terminating on a digital subscriber line access multiplexer (DSLAM). The ISP wants to allocate 800 Kbps of this link for video, if there is a video stream.

The ISP creates a classifier list to define a video packet flow, creates a policy to color the packets, and applies the policy to the interface: