PLANET XGS3-24042 - Manual (Page 657)

80-11
Common Session Parameters 0x0500
ATM Session Parameters 0x0501
Frame Relay Session Parameters 0x0502
Label Request Message ID 0x0600
Vendor-Private 0x3E00- 0x3EFF
Experimental 0x3F00- 0x3FFF
80.1.3 LDP Label Management
In the MPLS system, the downstream LSR determines the distribution of label to specific FEC, and notifies
the upstream. That is to say the labels are specified by the downstream and distributed from downstream to
upstream.
Label Advertisement Mode
In the MPLS domain, packets will be forwarded to the downstream LSR with the downstream LSR label after
the label switching process in the upstream LSR. The FEC labels distributed by the downstream LSR apply
only to itself and the upstream LSR, and should be advertised to the upstream LSR. MPLS defines two label
advertisement modes for the downstream LSR passing labels to its upstream LSR:
 DoD（Downstream On Demand）: LSR only distributes and advertises a label for the specified FEC
after receiving a label request message from the upstream.
 DU（Downstream Unsolicited）: LSR distributes and advertises a label for the specified FEC without
receiving a label request message from the upstream. It will automatically send label map
information and notify the upstream LSR.
These two modes can be mixed, with each LSR interface configured independently to use one of them.
During initialization, the upstream and downstream LSR have to exchange their label advertisement mode
information to reach an agreement on the mode. Otherwise the creation of LSP will fail.
The figure demonstrates the process of LDP label advertisement: