MPLS LDP: bindings vs bound addresses

[simone.c]

Hi all,
I would like to understand something about how LDP works. Let's imagine two routers, R1 & R2, LDP neighbors. Let's assume I am doing 'local label allocation control' on both routers. On R1 I have:

interface loopback99
ip address 99.99.99.99 255.255.255.255

and let's assume that due to label allocation control a label is not assigned to the prefix 99.99.99.99/32 (which is equivalent to say that this prefix is not found when checking show mpls ldp bindings).
Where things get confusing to me is when on R2, if I issue the command show mpls ldp neighbor {IP of R1}, I will see the prefix 99.99.99.99 (but no label assigned, again thanks to label allocation control):

show mpls ldp neighbor x.x.x.x
...
State: Oper; Msgs sent/rcvd: 667080/667741; Downstream
Up time: 1y5w
LDP discovery sources:
...
Addresses bound to peer LDP Ident:
99.99.99.99

Why is that so? Why is R1 informing R2 about this prefix if R1 is not even creating a binding for this prefix (thanks to label allocation control)?
If I read the Cisco documentation, it always talks about LDP advertising labels (e.g. By default, LDP advertises labels for all the prefixes to all its neighbors); but in this specific scenario I have described, LDP is advertising a prefix and not a label.
What is the rationale behind this behavior?
Thanks!

[Harold Ritter · Accepted solution]

Hi @simone.c ,

LDP neighbors advertise the list of directly connected interface addresses to each other. This list is what is shown in the "Addresses bound to peer LDP Ident:" of the "show mpls ldp neighbor". This is described in RFC5036 section 3.5.5.

The reason for this list is so the neighbor can map between the next hop address received via the routing protocol and the LDP identifier, as described in section 2.7 of the same RFC. So if a router receive the same prefix from several neighbors, the label used to that prefix will depend on that mapping between the next hop and LDP identifier.

This address list has nothing to do with label binding and whether a label is advertised for a specific local address or not.

https://www.rfc-editor.org/rfc/rfc5036.html

Regards,

[simone.c]

Hi Harold,
thanks for your detailed explanation. I've learnt something new! 
Regards,
Simone