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 ...