User Manual

80-12
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:
 Ordered Mode: For a FEC label mapping of a LSR, the LSR only advertise the mapping to its
upstream when it already has the label mapping of the FEC next-hop, or when it is the egress router
of the FEC. The label advertisement of a flow starts from the egress router of this FEC flow, binding
routers from downstream to upstream, thus to guarantee the mapping between labels and the flow is
complete and coherent in the whole network. The ordered mode can prevent loop more effectively.
 Independent Mode: LSR doesn’t have to wait for the label of the FEC next-hop to advertise labels to
its peer. It can notify label mapping to the LSR connected to it at any time. This mode may cause the
LSR advertise a label to its upstream before receiving one from its downstream. This mode can
accelerate the creation and aggregation of LSP.
Requirements for LSR to be an Egress router:
 The FEC quotes the LSR address;
 The FEC next-hop router locates outside the label switching network;
 The FEC unit passes the route area, such as another OSPF SUMMAERY domain, or another
autonomy system of OSPF, BGP.