Alcatel-Lucent 7210 SAS M - LDP Architecture

Label Distribution Protocol
7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide Page 221
When an unlabeled packet ingresses the 7210 SAS M 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, DUS, 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 24. 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.
Communication within LDP tasks is typically done by inter-process communication through the
event queue, as well as through updates to the various data structures. The primary data structures
that LDP maintains are:
• FEC/label database — This database contains all the FEC to label mappings that include,
both sent and received. It also contains both address FECs (prefixes and host addresses) as
well as service FECs (L2 VLLs and VPLS).
• Timer database — This database contains all the timers for maintaining sessions and
adjacencies.
• Session database — This database contains all the session and adjacency records, and
serves as a repository for the LDP MIB objects.