Default MPLS QoS Behavior in Cisco IOS

In Cisco IOS, the default behavior when imposing one or more labels on an IP packet is to copy the precedence value to the EXP bits of all imposed labels. This is called TOS reflection, because nothing regarding QoS changes by default. If, however, the six bits of the DSCP field are used, only the first three bits of DSCP are copied to the EXP bits of the labels. This leads to the first MPLS QoS rule.

MPLS QoS Rule 1: By default, in Cisco IOS, the precedence bits or the first three bits of the DSCP field in the IP header are copied to the EXP bits of all imposed labels at the ingress LSR.

Forwarding a labeled packet is a bit more complicated. You have to distinguish two cases: swapping a label, and possibly imposing one or more labels on the one hand and disposing one or more labels on the other hand. In the case of swapping an incoming label with an outgoing label on the LSR, the EXP bits are copied from the incoming label to the outgoing label. The same is true when swapping a label and then imposing one or more labels. The value of the EXP bits is copied from the incoming label to the swapped outgoing label and also to the labels that are then pushed on top of the swapped label. However, forwarding a labeled packet in the disposition mode, meaning that a label is popped, is quite different. When the router forwards a labeled packet and issues a pop, the value of the EXP bits is not copied to the newly exposed label or to the IP Precedence field (or possibly the DSCP field) of the IP header, if the packet becomes unlabeled. This means that, by default, in Cisco IOS, the EXP bits of the newly exposed label or the IP precedence/DSCP field of the newly exposed IP header remains unchanged and dictate the new QoS of the packet. This leads to the second, third, and fourth MPLS QoS rule.

MPLS QoS Rule 2: By default, in Cisco IOS, the EXP bits of the incoming top label are copied to the swapped outgoing label and to any label pushed onto that.

MPLS QoS Rule 3: By default, in Cisco IOS, the EXP bits of the incoming top label are not copied to the newly exposed label when the incoming label is popped.

MPLS QoS Rule 4: By default, in Cisco IOS, the EXP bits of the incoming top label are not copied to the precedence bits or DSCP bits when the label stack is removed and the IP header becomes exposed.

Furthermore, when you use MQC to change the QoS of the labeled packet, only the top label and possible pushed labels receive the new value for the EXP bits. The labels underneath the top label in the label stack do not receive the new value for the EXP bits. The precedence bits or DSCP bits do not change either. This means that when you manually change the QoS of a labeled packet on a certain LSR, that packet changes its QoS value sometime later in the network. Namely, when the label is popped off, the value of the EXP bits of the incoming top label is not copied to the newly exposed label, following MPLS QoS Rule 3. That means that the old QoS value of the packet is now active again. This actually happens frequently, because penultimate hop popping (PHP) is the default behavior in Cisco IOS. It happens by default in many MPLS networks and is something to watch out for. If you want to keep the new QoS value of the packet, you need to configure Cisco IOS commands to make sure that the QoS is copied down at disposition time. You will read more about that later in this chapter. This leads to the fifth MPLS QoS rule.

MPLS QoS Rule 5: When you change the EXP bits value through configuration, the value of the EXP bits in labels other than the top label, the swapped label, or the imposed labels and the precedence bits or DSCP bits in the IP header remain unchanged.

MPLS QoS Rules 4 and 5 lead to the fact that QoS tunneling becomes available. This means that the QoS value of the IP packet is transported through the MPLS network without change.

Look at Figure 12-5. You can see a Label Switch Router (LSR) with two interfaces. The left interface is the ingress interface, and the right is the egress interface. An IP packet or labeled packet comes in on the left interface and leaves the router on the right interface. This chapter uses this simple drawing throughout to clearly show the behavior of the EXP bits when a packet goes through the router. In each figure, the IP packet is the rectangle in the clear, and a label is one gray rectangle on top of the IP packet. In the IP packet, the QoS information is shown as the DSCP value. Each label also shows the EXP bits value.

Figure 12-5 Sample LSR Drawing with an Ingress and an Egress Interface

Label 1 Label 2

IP Packet

DSCP=5

Ingress Interface

Egress Interface

Label Switch Router

Figure 12-6 shows you the default treatment by Cisco IOS of the EXP bits in the label(s) when imposing, swapping, or disposing of labels. When you imply the five MPLS QoS rules, you get the default forwarding treatment that is shown in Figure 12-6.

Figure 12-6 Default Treatment by Cisco IOS of the EXP Bits When Imposing, Swapping, and Disposing of MPLS Labels

Imposition Imposition Swap and Imposition

Imposition Imposition Swap and Imposition

Swap and Imposition Swap

Swap and Imposition Swap

Disposition Disposition

Disposition Disposition

The first two pictures show you TOS reflection. By default, the IP precedence (or first three bits of DSCP) is copied to the imposed labels. This is MPLS QoS Rule 1. The third picture shows you that the EXP bits of the top label of the incoming packet are copied to the swapped label and the pushed label(s). This is MPLS QoS Rule 2. The fourth and fifth pictures are again examples of MPLS QoS Rule 2, but now they show that the EXP bits of the labels, which are below the top label at the ingress interface, are not changed (MPLS QoS Rule 5). The sixth picture shows you an example of MPLS QoS Rule 3, and the seventh picture is an example of MPLS QoS Rule 4.

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