What is the range of label values? What label values are reserved? What do the reserved values signify?
A. These values can also be found in RFC3032 – MPLS Label Stack Encoding leavingcisco.com. Theoretically, the range is 0 through (220-1). Label values 0-15 are reserved, and values 4-15 are reserved for future use. Values 0-3 are defined as: • A value of 0 represents the “IPv4 Explicit NULL Label”. This label indicates that the label stack must be popped, and the packet forwarding must be based on the IPv4 header. This helps to keep Exp bits safe until the egress router. It is used in MPLS based QoS. • A value of 1 represents the “Router Alert Label”. When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual packet forwarding is determined by the label beneath it in the stack. However, if the packet is forwarded further, the Router Alert Label should be pushed back onto the label stack before forwarding.
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