MPLS Label Allocation


In the Junos OS, label values are allocated per router or switch—the rest of this explanation uses router to cover both. The display output shows only the label (for example, 01024). Labels for multicast packets are independent of those for unicast packets. Currently, the Junos OS does not support multicast labels.

Labels are assigned by downstream routers relative to the flow of packets. A router receiving labeled packets (the next-hop router) is responsible for assigning incoming labels. A received packet containing a label that is unrecognized (unassigned) is dropped. For unrecognized labels, the router does not attempt to unwrap the label to analyze the network layer header, nor does it generate an Internet Control Message Protocol (ICMP) destination unreachable message.

A packet can carry a number of labels, organized as a last-in, first-out stack. This is referred to as a label stack. At a particular router, the decision about how to forward a labeled packet is based exclusively on the label at the top of the stack.

Figure 1 shows the encoding of a single label. The encoding appears after data link layer headers, but before any network layer header.

Figure 1: Label Encoding
Label Encoding

Figure 2 illustrates the purpose of the class-of-service bits (also known as the EXP or experimental bits). Bits 20 and 21 specify the queue number. Bit 22 is the packet loss priority (PLP) bit used to specify the random early detection (RED) drop profile. For more information about class of service and the class-of-service bits, see Configuring Class of Service for MPLS LSPs.

Figure 2: Class-of-Service Bits
Class-of-Service Bits

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