Cisco 3845 Software Guide - Page 500

Understanding QoS Chapter 27 Configuring QoS By choosing different values for the MPLS experimental field, you can mark packets based on their characteristics, such as rate or type, so that packets have the priority that they require during periods of congestion. Figure 27-4 shows an MPLS network that connects two sites of an IP network that belongs to a customer. Figure 27-4 MPLS Network Connecting Two Customer Sites Located at customer site Located at customer site Host A MPLS network IP network PE1 CE1 PE-CLE P1 MPLS network IP network PE2 P2 PE-CLE CE2 Host B 122012 Owned by service provider PE1 and PE2 are customer-located routers at the boundaries between the MPLS network and the IP network and are the ingress and egress provider-edge devices. CE1 and CE2 are customer edge devices. P1 and P2 are service provider routers within the core of the service-provider network. Packets arrive as IP packets at PE1, the ingress provider-edge router, and PE1 sends the packets to the MPLS network as MPLS packets. Within the service-provider network, there is no IP precedence field for the queuing mechanism to look at because the packets are MPLS packets. The packets remain as MPLS packets until they arrive at PE2, the egress provider-edge router. PE2 removes the label from each packet and forwards the packets as IP packets. Service providers can use MPLS QoS to classify packets according to the type, input interface and other factors by setting (marking) each packet within the MPLS experimental field without changing the IP precedence or DSCP field. You can use the IP precedence or DSCP bits to specify the QoS for an IP packet and use the MPLS experimental bits to specify the QoS for an MPLS packet. In an MPLS network, configure the MPLS experimental field value at PE1 (the ingress router) to set the QoS value in the MPLS packet. It is important to assign the correct priority to a packet. The packet priority affects how the packet is treated during periods of congestion. For example, service providers have service-level agreements with customers that specify how much traffic the service provider has agreed to deliver. To comply with the agreement, the customer must not send more than the agreed-upon rate. Packets are considered to be in-rate or out-of-rate. If there is congestion in the network, out-of-rate packets might be dropped more aggressively. MPLS QoS matches only valid MPLS packets. On input, the match is performed before any label processing on the packet. On output, the match is performed on the final packet after all label operations are performed. See the "Configuring MPLS and EoMPLS QoS" section on page 27-53. 27-12 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01