I have already make tiny lab with xconnect, but my curiosity has been recently exacerbated so that I can see exactly how it works…

What occurs behind the scene when you press ENTER (the reflexive command has already been pushed on R4) ?

R6(config-if)# xconnect 4.4.4.4 1111 encapsulation mpls

You can see on R4 :

R4#debug mpls ldp targeted-neighbors
LDP Directed Adjacency changes debugging is on
R4#debug mpls ldp transport connections 
LDP transport connection events debugging is on
R4#debug mpls ldp transport events 
LDP transport events debugging is on
[...]
*Aug 17 22:22:17.663: ldp: Peer LDP Id set to 6.6.6.6:0 for trgt 6.6.6.6, lcl addr = 4.4.4.4
*Aug 17 22:22:17.667: ldp: Rcvd ldp dir hello to 4.4.4.4 from 6.6.6.6 (6.6.6.6:0); FastEthernet0/1; opt 0xF
*Aug 17 22:22:17.671: ldp: ldp Hello from 6.6.6.6 (6.6.6.6:0) to 4.4.4.4, opt 0xF
*Aug 17 22:22:17.671: ldp: New directed adjacency 0x67A32E68 to 4.4.4.4 from 6.6.6.6 (6.6.6.6:0)
*Aug 17 22:22:17.675: ldp: Immediately request dhcb send hello back from 4.4.4.4 to 6.6.6.6
*Aug 17 22:22:17.675: ldp: local idb = targeted, holdtime = 90000, peer 6.6.6.6 holdtime = 90000
*Aug 17 22:22:17.675: ldp: dhcb intvl mbr cnt = 1, intvl = 10000, target = 6.6.6.6
*Aug 17 22:22:17.679: ldp: Opening listen port 646 for 6.6.6.6, 6.6.6.6
*Aug 17 22:22:17.683: ldp: No MD5 password protection for peer 6.6.6.6:0
*Aug 17 22:22:17.683: ldp: Registered TCB with LDP TCB database tcb 0x66BB49A0 [key
R4# 1779], total 2
*Aug 17 22:22:17.683: ldp: Open LDP listen TCB 0x66BB49A0; lport = 646; fhost = 6.6.6.6; with normal priority
*Aug 17 22:22:17.683: ldp: Add listen TCB to list; tcb 0x66BB49A0 [key 1779]; addr 6.6.6.6
*Aug 17 22:22:17.683: ldp: Send ldp dir hello; no idb, src/dst 4.4.4.4/6.6.6.6, inst_id 0
*Aug 17 22:22:18.027: ldp: Rcvd ldp dir hello to 4.4.4.4 from 6.6.6.6 (6.6.6.6:0); FastEthernet0/1; opt 0xF
*Aug 17 22:22:18.027: ldp: ldp Hello from 6.6.6.6 (6.6.6.6:0) to 4.4.4.4, opt 0xF
*Aug 17 22:22:18.027: ldp: local idb = targeted, holdtime = 90000, peer 6.6.6.6 holdtime = 90000
*Aug 17 22:22:18.027: ldp: dhcb intvl mbr cnt = 1, intvl = 10000, target = 6.6.6.6
*Aug 17 22:22:18.043: ldp: Registered TCB with LDP TCB database tcb 0x66BB4FDC [key 1780], total 3
*Aug 17 22:22:18.047: ldp: Incoming ldp conn 4.4.4.4:646  6.6.6.6:38742; with normal priority
*Aug 17 22:22:18.051: ldp: Found adj 0x67A32E68 for 6.6.6.6 (Hello xport addr opt)
*Aug 17 22:22:18.051: ldp: New t
R4#emporary adj 0x66BB5618 from 6.6.6.6
*Aug 17 22:22:18.055: ldp: Real adj 0x67A32E68 bound to 6.6.6.6:0, replacing temp adj 0x66BB5618
*Aug 17 22:22:18.059: ldp: Adj 0x66BB5618; state set to closed
*Aug 17 22:22:18.183: ldp: Data received!
*Aug 17 22:22:18.187: ldp: : peer 6.6.6.6:0 down reason reset to None
*Aug 17 22:22:18.187: %LDP-5-NBRCHG: LDP Neighbor 6.6.6.6:0 (2) is UP
*Aug 17 22:22:18.191: ldp-trgtnbr: 6.6.6.6 Received address addition notif start; flags 0x13
*Aug 17 22:22:18.195: ldp-trgtnbr: 6.6.6.6 Set peer start; flags 0x13
*Aug 17 22:22:18.195: ldp-trgtnbr: 6.6.6.6 Set peer finished; flags 0x1F
*Aug 17 22:22:18.195: ldp-trgtnbr: 6.6.6.6 Received address addition notif finish; flags 0x1

One LDP targeted session is build to establish the xconnect session. This targeted session is possible thanks to IGP (here OSPF).

You could read the pcap beetween R6 and R3 I have recorded. Really interesting.

http://www.cloudshark.org/captures/6e68003ead16

You can see the double MPLS tag, one for the xconnect P2P link established with R4 for the VC-ID 1111 and one to route the label through the MPLS cloud.

I hope this can help someone. For my part, I have had fun to do this tiny lab.