When implementing QoS over MPLS infrastructure, the Edge LSR between the IP and MPLS domains performs translation from the IP QoS domain to the MPLS QoS domain or vice versa. Specifically, an IP packet can enter a MPLS domain (for example, CE-PE); this is called the IP2MPLS condition. In some cases, the packet can be applied a label stack, and one of the labels in the label stack might have the EXP bit manipulated; this is called a MPLS2MPLS condition. Finally, a labeled packet might be converted to an appropriate IP packet (as in the case of an egress LSR in PE-CE conditions), and this is referred to as the MPLS2IP condition.
In the IP2MPLS condition, the IP packet is given a MPLS label. During this procedure, if the incoming IP packet has an associated IP Precedence, this value is copied to the MPLS EXP field, as shown in Figure 13-5. Cisco routers copy IP Precedence bits from the L3 header into MPLS EXP bits on label imposition when functioning as an ingress PE router.
Figure 13-5. MPLS QoS Implementation and Functions
In the MPLS2MPLS condition, the incoming packet is a labeled packet. Therefore, in the MPLS2MPLS condition, three actions, namely Push, Pop, or Swap, can be performed. As illustrated in Figure 13-5, in the MPLS2MPLS Push condition, an incoming labeled packet is given another label. In the MPLS2MPLS Pop operation, the incoming labeled packet is stripped off the top-most label in the label stack, resulting in a labeled packet. In the swap condition, an incoming labeled packet's top-most label is swapped with a new label. Cisco routers by default copy the top label's EXP value onto the underlying label in a label stack during label disposition in an MPLS2MPLS condition.
In the MPLS2IP condition, the incoming packet is a labeled packet and the outgoing packet is a pure IP packet. In the MPLS2IP condition, the EXP value is copied back onto the underlying IP Precedence value of the IP packet depending on the tunnel mode implementation in use. The different tunnel mode implementations are explained in the next section. All three conditions discussed have been illustrated in Figure 13-5.