a. Piggyback the label information on to the back of an existing routing protocol.
b. Use a distribution protocol such as LDP (Label Distribution Protocol).
The first method is to piggy-back label distribution information onto the routing protocol. This can be quite a practical method providing the routing protocol supports this distribution method. This method is probably the simplest as it does not involve any additional protocols and the routing protocols is already in use.
The second method is to use a dedicated Label Distribution Protocol (LDP), which has been designed to perform all the functions necessary to ensure the correct labels have populated the Label Table. The Label Switched Routers (LSRs) have a discovery method to discover the presence of each other through the use of 4 specific message types:
- LDP Discovery Messages
- LDP Adjacency messages to initialise, and shutdown peer connections.
- LDP Label Advertisement Messages to advertise, request, withdraw and release abels.
- LDP Notification messages, which are either advisory or contain error information.
A Label Switched Router will periodically transmit Hello messages via well known UDP port number 646 to all routers part of the LDP multicast group. Once communication has been made, TCP will be used reliably in a bi-directional manner. If a TCP connection cannot be made, and therefore label binding not occur, then the only option for the router is to use conventional routing methods.
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