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LDP Basic Discovery Mechanism

To discover directly connected peers, LSRs periodically send out LDP link hellos on the interface. The link hellos are contained in UDP packets that are addressed to the well-known LDP discovery port, 646. The destination address for the ports is 224.0.0.2. Using this port and address ensures that the hellos are sent to all routers on the interface’s subnet.

The link hello includes the LDP identifier for the label space that the LSR intends to use for the interface. In the JUNOSe implementation, this is always the platform label space, so the LDP identifier specifies the LSR ID and a value of 0 for the label space. The link hello also includes other information, such as the hello hold time configured on the interface. The hello hold time specifies how long an LSR maintains a record of hellos received from potential peers.

When an LSR receives a link hello, it identifies the sending LSR as a potential LDP peer on that interface. The LSRs form a hello adjacency to keep track of each other.

The basic discovery mechanism is enabled by default when you enable LDP on an interface. You can configure the link hellos in the LDP profile with the hello hold-time and hello interval commands. You can configure a transport IP address to be globally included in link hellos with the mpls ldp discovery transport-address command.

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