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Figure 9 Dynamically establishing an LSP
Label distribution and control
Label advertisement modes
Figure 10 Label advertisement modes
LDP advertises label-FEC mappings in one of the following ways:
• Downstream Unsolicited (DU) mode—Distributes FEC-label mappings to the upstream LSR,
without waiting for label requests. The device supports only the DU mode.
• Downstream on Demand (DoD) mode—Sends a label request for an FEC to the downstream
LSR. After receiving the label request, the downstream LSR distributes the FEC-label mapping
for that FEC to the upstream LSR.
A pair of upstream and downstream LSRs must use the same label advertisement mode. Otherwise,
the LSP cannot be established.
LSR A
LSR B LSR D
LSR C
LSR E
LSR F LSR G
LSR H
Ingress
Egress
LSP
Label mapping
Egress
Transit
DU mode
DoD mode
2) Sends a label request for the
FEC to the downstream.
3) Distributes a label mapping for
the FEC to the upstream upon
receiving the request.
Ingress
1) Unsolicitedly distributes a label
mapping for a FEC to the
upstream.
2) Unsolicitedly distributes a label
mapping for the FEC to the
upstream.
1) Sends a label request for a
FEC to the downstream.
4) Distributes a label mapping for
the FEC to the upstream upon
receiving the request.
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