Default MPLS QoS Behavior in Cisco IOS

In Cisco IOS, the default behavior when imposing one or more labels on an IP packets 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.

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

This means that the QoS value of the IP packet is transported through the MPLS network without change. No matter how many times the EXP bits are changed, by default, the IP precedence or DSCP bits of the IP packet are preserved; the value at the egress LSR is the same as when the IP packet entered the MPLS network.

You can now tunnel the DiffServ value of the IP packet through the MPLS network(hence the name DiffServ Tunneling). The IETF has defined three models to tunnel the DiffServ information. All three models are distinct and have their own merits. Furthermore, the distinction between the three models is only at the edge LSRs. The P routers do not come into play with regard to the different DiffServ tunneling models.

• Pipe Model
• Short Pipe Model
• Uniform Model