Highest loopback id starts the Label Distribution Protocol initialization process by sending common session parameter TLV which includes a sub TLV of parameters containing session protocol version, session keepalive time, advertisement method, loop detection and session path vector. In the given diagram, I have started MPLS LDP firstly on 100.100.100.1 (R1) and 100.100.100.2(R2). So in this case R2 send a first initialization message to R1 by adding all the above TLV parameters.
Structure of LDP initialization message sent from R2 to R1.
Figure 1 (Click on image to see in new window)
Being R2 is having highest LDP router id, so it start sending the label information to R1. We can say R2 is working as downstream LSR to R1. R2 sending Address message and label mapping message which are sub part of label distribution protocol.
Figure 2(Click on image to see in new window)
Address message is only containing the directly connected interface ip address of R2 which are 22.214.171.124, 126.96.36.199 and 100.100.100.2.
Figure 3(Click on image to see in new window)
Along with this R2 is also sending label mapping message which is containing the information about route, label and address family. Address family means whether it is ipv4 route or vpnv4 route. R2 is sending a information about 188.8.131.52 prefix by including label 3 which is used for PHP (Penultimate Hop Popping).
Figure 4(Click on image to see in new window)
The same way R2 is sending a label 16 to R1 for 100.100.100.1 which is loopback address of R1 itself and label 16 becomes as local label in R2 forwarding table.
Figure 5(Click on image to see in new window)
Figure 6(Click on image to see in new window)
Once this is done then R1 initiates a label mapping process by sending the label and FEC information to R2 ( R2 is upstream LSR to R1).
After this, R3 is started which is having loopback address of 100.100.100.3. In this case R3 will start the initialization process of LDP being R3 is having highest LDP router id.
Figure 7(Click on image to see in new window)
Now R3 becomes downstream LSR to R2 and R2 becomes upstream LSR to R3. Once R3 will exchange all the label mappings then R2 will initiate the process and send the information to R3.
Now how the updates of new route will flood. To test this, a new route 100.100.100.100 is installed in R3. After this we saw a label mapping message was sent from 100.100.100.2 to 100.100.100.1 and vice versa but the communication was being started by 100.100.100.2
Figure 8(Click on image to see in new window)
How does LDP sends new updates to its peer?
LDP uses label mapping message to send the partial label updates to its peer.
Label mapping message is containing the information of new route 100.100.100.100 with label 18.
Figure 9(Click on image to see in new window)