4000 Router Series MPLS Configuration Guide

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—Unsolicitedly 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 a 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.

NOTE:

pair of upstream and downstream LSRs must use the same label advertisement mode. Otherwise, the

LSP cannot be established.

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) Unsolicitely distributes a label

mapping for a FEC to the upstream.

1) Sends a label request for a

FEC to the downstream.

2) Unsolicitely distributes a label

mapping for the FEC to the

upstream.

4) Distributes a label mapping for

the FEC to the upstream upon

receiving the request.