MPLS Label Distribution and Signaling
(continued)

DOD

When labels are requested, they are “pulled” down or demanded, so this method has been called pulled or down-stream on demand (DOD).  Note in Figure 5, that in the first step   the labels are requested and in the second step the labels are sent.

Figure 5:  Down-stream on Demand  (DOD)

Whether the labels arrive via independent or ordered control, or via DOD or DOU, the label switch router (LSR) creates a cross-connect table similar to the one shown in Figure 6.

The connect tables are sent to router r3 to r1. The tables heading read:  label-in, port-in, label-out, port-out, and instruction (I).  In this case, the instruction is to swap (s).  It is important to note that the labels and cross-connect tables are router specific.

After the cross-connect tables are loaded, the data can flow from router 1 to router 4 with each router following its instructions to swap the labels.

Figure 6: LSR with Cross-connect Tables Populated

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