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Advertisement message: Used to create, alter, or remove label bindings.
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Notification message: Used to provide advisory information and to notify errors.
For reliable transport of LDP messages, TCP is used for LDP session messages, advertisement
messages, and notification messages, while UDP is used only for discovery messages.
Label space and LDP identifier
A scope of labels that can be assigned to LDP peers is called a label space. A label space can be per
interface or per platform. A per interface label space is interface-specific, while a per platform label
space is for an entire LSR.
An LDP identifier is used to identify an LSR label space. It is a six-byte numerical value in the format of
<LSR ID>:<Label space ID>, where LSR ID is four-byte long. A label space ID of 1 means that the label
space is per interface, a label space ID of 0 means that the label space is per platform.
Currently, only per platform label space is supported.
LDP Label Distribution
Figure 1-7 illustrates how LDP distributes labels.
Figure 1-7
Label distribution
LER
LSR A
LSR B LSR D
LSR C
LSR E
LSR F LSR G
LSR H
Ingress
Egress
Label request
LSP1
LSP2
Label mapping
In Figure 1-7 , B is the upstream LSR of C along LSP 1.
As described previously, there are two label advertisement modes. The main difference between them
is whether the downstream advertises the bindings unsolicitedly or on demand.
The following details the advertisement process in each of the two modes.
DoD mode
In DoD mode, an upstream LSR sends a label request message containing the description of a FEC to
its downstream LSR. After receiving the message, the downstream LSR assigns a label to the FEC,
encapsulates the binding information in a label mapping message and sends the message to the