 are created in data voice priority. Routers and layer 3 data switches can then switches. A voice VLAN can also be manually applied to use these ports to distinguish priority traffic. This pri- an IP telephone or given by a DHCP server. CoS tagging ority traffic can be voice packets (UDP), signaling and QoS policies can be applied at OSI layer 2 by using packets (TCP), or both. This is an OSI model layer 4 VLANs. VolP Technology CHAPTER 3.3 Separate voice and data VLANs are options that make application is interested in the traffic flow offered by the sense for many users. Note, however, that VLAN sender). The RESV message winds its way back toward implementation and maintenance can be substantial, and the sender, following the path established by the incident again, is an option even as a best practice. PATH messages. As the RESV message progresses toward the sender, RSVP-aware devices verify that they Resource reservation protocol have the resources necessary to meet the QoS re- quirements requested. If a device can accommodate the Once a given packet has been labeled by one or more of resource request, it installs classification state corre- the schemes already mentioned, there are two common sponding to the conversation and allocates resources for methods of acting on those labels to give the voice the conversation. The device then allows the RESV packets priority. One method is to act on a per-packet message to progress on toward the sender. If a device basis by giving special treatment to the packets as they cannot accommodate the resource request, the RESV are processed. The second is to establish a specialized message is rejected and a rejection is sent back to the route for those packets to traverse in advance. receiver. In this section we will talk about the most common In addition, RSVP-aware devices in the data path may protocol used to support that second method, the RSVP. extract policy information from PATH messages and/or RSVP is a signaled QoS configuration mechanism. It is RESV messages, for ver