Label Stack Treatment
As mentioned earlier in this book, MPLS has 3 EXP bits in the label header that are used in much the same way as IP Precedence bits or the DSCP CS bits.
You should consider three cases. The first case is when IP packets enter an MPLS network. This is known as the ip-to-mpls case, often written as ip2mpls.
The second case is when packets that already have one or more labels have their label stack manipulated. This is known as the mpls-to-mpls case, or mpls2mpls.
The third case is when packets exit an MPLS network and have all their labels removed. This is known as the mpls-to-ip path, or mpls2ip.
ip2mpls
Packets entering an MPLS network have one or more labels applied to an underlying IP packet. This is an ip2mpls push,
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