Label Distribution Protocol
7450 ESS MPLS Guide Page 519
single LDP session allows each peer to learn the other's label mappings (LDP is bi-directional)
and to exchange label binding information.
LDP signaling works with the MPLS label manager to manage the relationships between labels
and the corresponding FEC. For service-based FECs, LDP works in tandem with the Service
Manager to identify the virtual leased lines (VLLs) and Virtual Private LAN Services (VPLSs) to
signal.
An MPLS label identifies a set of actions that the forwarding plane performs on an incoming
packet before discarding it. The FEC is identified through the signaling protocol (in this case,
LDP) and allocated a label. The mapping between the label and the FEC is communicated to the
forwarding plane. In order for this processing on the packet to occur at high speeds, optimized
tables are maintained in the forwarding plane that enable fast access and packet identification.
When an unlabeled packet ingresses the router, classification policies associate it with a FEC. The
appropriate label is imposed on the packet, and the packet is forwarded. Other actions that can take
place before a packet is forwarded are imposing additional labels, other encapsulations, learning
actions, etc. When all actions associated with the packet are completed, the packet is forwarded.
When a labeled packet ingresses the router, the label or stack of labels indicates the set of actions
associated with the FEC for that label or label stack. The actions are preformed on the packet and
then the packet is forwarded.
The LDP implementation provides DOD, DU, ordered control, liberal label retention mode
support.
LDP Architecture
LDP comprises a few processes that handle the protocol PDU transmission, timer-related issues,
and protocol state machine. The number of processes is kept to a minimum to simplify the
architecture and to allow for scalability. Scheduling within each process prevents starvation of any
particular LDP session, while buffering alleviates TCP-related congestion issues.
The LDP subsystems and their relationships to other subsystems are illustrated in Figure 47. This
illustration shows the interaction of the LDP subsystem with other subsystems, including memory
management, label management, service management, SNMP, interface management, and RTM.
In addition, debugging capabilities are provided through the logger.
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