/mcpQuality of Service for Voice over IP Packet Classification 4 QoSVoIP.mif • Protocol (such as URLs, stateful protocols, or Layer 4 protocol) • Input port • IP Precedence or DSCP • Ethernet 802.1p class of service (CoS) As mentioned, it can be processor-intensive if nodes must repeat classification based on access list matches. Therefore, nodes should mark packets as soon as they have identified and classified the VoIP packets. If a node can set the IP Precedence or DSCP bits in the ToS byte of the IP header as soon as it identifies traffic as being VoIP traffic, then all of the other nodes in the network can classify based on these bits. Marking is the process of the node setting one of the following: • Three IP Precedence bits in the IP ToS byte • Six DSCP bits in the IP ToS byte • Three MPLS Experimental (EXP) bits • Three Ethernet 802.1p CoS bits • One ATM cell loss probability (CLP) bit In most IP networks, marking IP Precedence or DSCP should be sufficient to identify traffic as VoIP traffic. Voice Dial Peers Classification and Marking Example With Cisco VoIP gateways, you typically use voice dial peers to classify the VoIP packets and mark the IP Precedence bits. The following configuration example shows how to mark the IP Precedence bits: Committed Access Rate Classification and Marking Example Committed access rate (CAR) is an older technique that involves rate-limiting or policing traffic that matches certain criteria to an upper bound. CAR supports most of the matching mechanisms and allows IP Precedence or DSCP bits to be set differently depending on whether packets conform to or exceed a specified rate. Configuration Example 1: Classification and Marking Using Dial Peers dial-peer voice 100 voip destination-pattern 100 session target ipv4:10.10.10.2 ip precedence 5 In this example, any VoIP call that matches the dial-peer voice 100 voip command will have all of its voice payload packets set with IP Precedence 5—meaning that the three most significant bits of the IP ToS byte are set to 101.