PLANET XGS3-24042 - Manual (Page 658)

80-12
Figure 80-3 The Process of Label Advertisement
For example, as for LSP1 in the above figure, LSR B is the upstream LSR of LSR C, while LSR C is the
downstream LSR of LSR B.
The main difference of two label advertisement mode lies on whether the label advertisement is DoD or DU.
The following is the detailed label advertisement process of these two modes:
（1） DoD（downstream-on-demand）
The upstream LSR send a Label Request Message, which carries FEC description to its downstream LSR.
The downstream LSR will distribute a label for this FEC, and respond to the upstream with the mapped label
via a Label Mapping Message.
When will the downstream LSR respond the Label Mapping Message depends on the label advertisement
mode it adopted.
1) In Ordered mode, it will send the Label Mapping Message to the upstream only after receiving a Label
Mapping Message from its downstream.
2) In Independent mode, it will immediately send the Label Mapping Message to the upstream no matter it
receives a Label Mapping Message from its downstream or not.
Usually, the upstream LSR chooses the downstream LSR based on the routing table. In the above figure, all
LSR along the LSP1 work in the ordered mode, while LSR F in LSP2 in the Independent mode.
（2） DU（downstream unsolicited）
The downstream LSR will automatically advertise the label mapping message to its upstream LSR after the
LDP session successfully created. The upstream LSR will save the message and process it according to its
retention mode.
Label Distribution Control Mode
In the MPLS domain, LSR generate a LSP from the ingress router and the egress router via switching labels,
based on the route-forwarding path created by IGP in the MPLS domain. Only a complete path is useful for
pack forwarding. The creation of LSP is the LSR label advertisement process; hence, controlling the creation
of LSP is controlling the LSR label advertisement. MPLS defines two LSP control modes to determine when
the downstream LSR will advertise labels to the upstream LSR:
LSP1
MPLS LER
MPLS LSR
Ingress
Egress
LSP2
Label Request
Label Mapping
LDP Session
A B C
D
E
F
G
H