MPLS notes

Here is some MPLS notes
- In frame mode, MPLS uses a 32-bit label that is inserted between the Layer 2 and Layer 3 headers
- PHP (Penultimate Hop Popping): is performed in MPLS-based networks where the router upstream to the Edge LSR removes the top label in the label stack and forwards only the resulting packet (either labeled IP or IP packet) for a particular FEC
- Reserved Labels: Labels 0 through 15 are reserved labels. An LSR cannot use them in the normal case for forwarding packets. An LSR assigns a specific function to each of these labels. Label 0 is the explicit NULL label, whereas label 3 is the implicit NULL label. Label 1 is the router alert label, whereas label 14 is the OAM alert label. The other reserved labels between 0 and 15 have not been assigned yet.
+ Implicit Null (value =3): An egress LSR assigns the implicit NULL label to a FEC if it does not want to assign a label to that FEC, thus requesting the upstream LSR to perform a pop operation. Although the label value 3 signals the use of the implicit NULL label, the label 3 will never be seen as a label in the label stack of an MPLS packet. That is why it is called the implicit NULL label.
+ Explicit Null (value=0): In case of using EXP bit in MPLS header for QoS, the implicit Null cannot reserved this value since the last label is removed one-hop before (i.e.PHP). The Explicit Null has the same meaning with Implicit Null but it reserves the EXP value for QoS. If a LSR propagates an explicit-null label, the upstream LSR does not POP the label but assigns a label value of 0 and sends a labeled packet to that LSR.