Uniform Model

The Uniform model is quite different from the Pipe or Short Pipe model. In the Uniform model, the following rules apply:

■ The LSP DiffServ information must be derived from the Tunneled DiffServ information on the ingress LSR.

■ On an intermediate LSR (a P router), the LSP DiffServ information of the outgoing label is derived from the LSP DiffServ information of the incoming label.

■ On the egress LSR, the LSP DiffServ information must be propagated to the Tunneled DiffServ information.

Notice the change in the first bullet: The LSP DiffServ information must be derived from the Tunneled DiffServ information on the ingress LSR. On the egress LSR, the Tunneled DiffServ information is derived from the LSP DiffServ information. This means that a packet belongs to the same QoS class at any time. The QoS information is always present in the topmost label or in the IP header if the packet is not labeled. The MPLS network does not have an impact on the QoS information, but it does switch the packets through the MPLS network, of course.

You can instruct the router to change the EXP bits of the top label(s) through configuration (by using MQC in Cisco IOS) anywhere in the MPLS cloud. This only changes the outer QoS information, or the LSP DiffServ information. This change in the LSP DiffServ information is not propagated down to the Tunneled DiffServ information in the Pipe model and Short Pipe model on the egress LSR. It is, however, propagated on the egress LSR when you are using the Uniform model.

In Cisco IOS, the configuration that you need to enable one of the three DiffServ Tunneling models is MQC. MQC is configurable per interface. Therefore, you can choose the DiffServ Tunneling model per interface—hence, per customer connecting to the MPLS network. The only LSRs where this configuration is needed are the ingress and egress LSRs. On those routers, the specific DiffServ Tunneling model is determined.

Figure 12-8 shows you the three tunneling models in the case of an MPLS virtual private network (VPN). The packets that are entering and exiting the MPLS VPNs are IP packets. Therefore, the Tunneled DiffServ information translates to the DSCP bits in the IP header, and the LSP DiffServ information translates to the EXP bits in the MPLS labels.

Uniform Model

Ingress PE

Egress PE

Egress PE

EXP Bits Set According to Service Provider's Policy

Short Pipe Model

EXP Bits Set According to Service Provider's Policy

EXP = 3

EXP = 3

EXP = 3

EXP = 3

DSCP = 5

Scheduling and Discard Behavior Based on DSCP

Egress PE

EXP Bits Set According to Service Provider's Policy

Pipe Model

Pipe Model

EXP Bits Set According to Service Provider's Policy

EXP = 3

EXP = 3

EXP = 3

EXP = 3

DSCP = 5

DSCP = 5

Scheduling and Discard Behavior Based on EXP

Ingress PE

Egress PE

NOTE The MPLS DiffServ Tunneling models are described in RFC 3270, "Multi-Protocol Label Switching (MPLS) Support of Differentiated Services."

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