Label Operation

The possible label operations are swap, push, and pop. Look at Figure 3-1 to see the possible operations on labels.

Figure 3-1 Operations on Labels

SWAP PUSH

23 16

By looking at the top label of the received labeled packet and the corresponding entry in the LFIB, the LSR knows how to forward the packet. The LSR determines what label operation needs to be performed—swap, push, or pop—and what the next hop is to which the packet needs to be forwarded. The swap operation means that the top label in the label stack is replaced with another, and the push operation means that the top label is replaced with another and then one or more additional labels are pushed onto the label stack. The pop operation means that the top label is removed.

NOTE The LSR sees the 20-bit field in the top label, looks up this value in the LFIB, and tries to match it with a value in the local labels list.

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