Label Distribution Protocol
370
MPLS Guide
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For LDP label advertisement, DU mode is supported. To prevent filling the uplink
bandwidth with unassigned label information, Ordered Label Distribution Control
mode is supported.
A PW/VLL label can be dynamically assigned by targeted LDP operations. Targeted
LDP allows the inner labels (that is, the VLL labels) in the MPLS headers to be
managed automatically. This makes it easier for operators to manage the VLL
connections. There is, however, additional signaling and processing overhead
associated with this targeted LDP dynamic label assignment.
5.1.1.1 BFD for T-LDP
BFD is a simple protocol for detecting failures in a network. BFD uses a “hello”
mechanism that sends control messages periodically to the far end and receives
periodic control messages from the far end. BFD is implemented in asynchronous
mode only, meaning that neither end responds to control messages; rather, the
messages are sent in the time period configured at each end.
A T-LDP session is a session between either directly or non-directly connected peers
and requires that adjacencies be created between two peers. BFD for T-LDP
sessions allows support for tracking of failures of nodes that are not directly
connected. BFD timers must be configured under the system router interface context
before being enabled under T-LDP.
BFD tracking of an LDP session associated with a T-LDP adjacency allows for faster
detection of the status of the session by registering the loopback address of the peer
as the transport address.
5.1.2 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 22. 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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