Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 640

Terminology Chapter 39 Configuring MPLS QoS • Configuring Short Pipe Mode, page 39-34 • Configuring Uniform Mode, page 39-39 Terminology This section defines some MPLS QoS terminology: • Class of Service (CoS) refers to three bits in either an Inter-Switch Link (ISL) header or an 802.1Q header that are used to indicate the priority of the Ethernet frame as it passes through a switched network. The CoS bits in the 802.1Q header are commonly referred to as the 802.1p bits. To maintain QoS when a packet traverses both Layer 2 and Layer 3 domains, the type of service (ToS) and CoS values can be mapped to each other. • Classification is the process used for selecting traffic to be marked for QoS. • Differentiated Services Code Point (DSCP) is the first six bits of the ToS byte in the IP header. DSCP is only present in an IP packet. • E-LSP is a label switched path (LSP) on which nodes infer the QoS treatment for MPLS packets exclusively from the experimental (EXP) bits in the MPLS header. Because the QoS treatment is inferred from the EXP (both class and drop precedence), several classes of traffic can be multiplexed onto a single LSP (use the same label). A single LSP can support up to eight classes of traffic because the EXP field is a 3-bit field. The maximum number of classes would be less after reserving some values for control plane traffic or if some of the classes have a drop precedence associated with them. • EXP bits define the QoS treatment (per-hop behavior) that a node should give to a packet. It is the equivalent of the DiffServ Code Point (DSCP) in the IP network. A DSCP defines a class and drop precedence. The EXP bits are generally used to carry all the information encoded in the IP DSCP. In some cases, however, the EXP bits are used exclusively to encode the dropping precedence. • Frames carry traffic at Layer 2. Layer 2 frames carry Layer 3 packets. • IP precedence is the three most significant bits of the ToS byte in the IP header. • QoS tags are prioritization values carried in Layer 3 packets and Layer 2 frames. A Layer 2 CoS label can have a value ranging between zero for low priority and seven for high priority. A Layer 3 IP precedence label can have a value ranging between zero for low priority and seven for high priority. IP precedence values are defined by the three most significant bits of the 1-byte ToS byte. A Layer 3 DSCP label can have a value between 0 and 63. DSCP values are defined by the six most significant bits of the 1-byte IP ToS field. • LERs (label edge routers) are devices that impose and dispose of labels upon packets; also referred to as Provider Edge (PE) routers. • LSRs (label switching routers) are devices that forward traffic based upon labels present in a packet; also referred to as Provider (P) routers. • Marking is the process of setting a Layer 3 DSCP value in a packet. Marking is also the process of choosing different values for the MPLS EXP field to mark packets so that they have the priority that they require during periods of congestion. • Packets carry traffic at Layer 3. • Policing is limiting bandwidth used by a flow of traffic. Policing can mark or drop traffic. 39-2 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03