RFC 5036, "LDP Specification", October 2007Source of RFC: mpls (rtg)
Errata ID: 4512
Publication Format(s) : TEXT
Reported By: Jiessie
Date Reported: 2015-10-26
Rejected by: Deborah Brungard
Date Rejected: 2015-11-18
Section 2.5.2 says:
2.4.1. Basic Discovery Mechanism To engage in LDP Basic Discovery on an interface, an LSR periodically sends LDP Link Hellos out the interface. LDP Link Hellos are sent as UDP packets addressed to the well-known LDP discovery port for the "all routers on this subnet" group multicast address. 2.5.2. Transport Connection Establishment Note that when an LSR sends a Hello, it selects the transport address for its end of the session connection and uses the Hello to advertise the address, either explicitly by including it in an optional Transport Address TLV or implicitly by omitting the TLV and using it as the Hello source address.
According to 2.4.1, an LSR send Hellos to "all routers on this subnet", the eligible reception LSR should be on this subnet.
2.5.2 states that one way for an LSR to advertise Transport Address is to uses it as Hello source address.
On the topology below, LSRa uses 184.108.40.206 as Transport Address and sends Hello(source:220.127.116.11 destination:18.104.22.168) to LSRb.
LSRb will ignore this packet which is not on its interface subnet.
My question is:
What's the scenario to use "implicitly" way to send Hello?
This is not an errata, it is a question to be asked on the MPLS mailing list.