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Part 2: Special Labels in MPLS

Special Labels in MPLS 

Earlier we talked about the labels and header in MPLS where we defined that labels from 0-15 are reserved labels in MPLS space. Now the question is for what we are reserving these labels and what are the purpose of these labels?

Fig 1.1- MPLS Label

Let's talk about these labels ranging from 0-15 and the purpose 

Label Value 0: It is named as IPv4 Explicit NULL Label.  If the egress node allocates a label with the value of 0 to the penultimate hop LSR, the penultimate hop LSR pushes label 0 to the top of the label stack and forwards the packet to the egress node. When the egress node detects that the label of the packet is 0, the egress node pops the label out.

Label Value 1:It is a Router Alert Label. Router Alert label which is a label value 1 is only valid when it is not at the bottom of a label stack. If the packet needs to be forwarded continuously, the node pushes the Router Alert Label to the top of the label stack again.

Label Value 2: It is named as IPv6 Explicit NULL Label. The process is same as of Label value 0 but in the IPv6 environment.

Label Value 3: stated as Implicit NULL Label. When the label with the value of 3 is swapped on an LSR at the penultimate hop, the LSR pops the label out and forwards the packet to the egress node. Upon receiving the packet, the egress node forwards the packet in IP forwarding mode or according to the next layer label.

Label Value 4 to 13 : not used and is reserved for future purposes in case there is an enhancement in MPLS. 

Label Value 14: It states as OAM Router Alert Label. A label for operation, administration and maintenance (OAM) packets over an MPLS network. MPLS OAM sends OAM packets to monitor LSPs and report faults. OAM packets are transparent on transit nodes and the penultimate LSR.

Label Value 15: not used and is reserved for future purposes in case there is an enhancement in MPLS. 

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