I implore all my readers to always remember this topic: IGP LDP synchronization. It is important to use IGP LDP synchronization to avoid blackholing, especially when MPLS networks fails to function effectively.
In the topology above, IS-IS is running in the network of the service provider. For the transport label distribution or topmost label/tunnel label, LDP is used even though RSVP or segment routing could be used as well.
If any link fails in the MPLS networks, IGP will not converge on the failed link before getting green light from the LDP. Also, if P3-P4 link fails in the topology shown above, P1-P2-P4 link is used. If the link returns and if IGP converges before LDP comes together, P3 cannot create a label for the prefixes; it sends the regular IP packet to P4. In fact, P4 drops the packets because it cannot recognize the CE (Customer).
If IGP-LDP synchronization feature is enabled, P3 and P4 signals their neighbor not to use P3-P4 link unless LDP converges. BGP signals the IGP exactly in the same way. With OSPF max-metric and IS-IS overload bit, OSPF allows BGP to signal it.
LDP session protection is another way to prevent traffic blackholing if an MPLS enabled link fails to work properly. If you don’t know much about LDP session, don’t worry I will explain it in a E-VPN article very soon. Sometimes, one protocol can be responsible for many actions. For example, with segment routing, IGP prefix distribution and LDP label distribution use the same advertisement. So, you don’t need to use LDP as a separate label distribution protocol. Also, you don’t need to worry about LDP IGP synchronization or LDP session protection with SR.
In sum, the overlay protocols should follow and signal the underlay protocols. It can be LDP over IGP, BGP over IGP, HSRP over Spanning Tree, LISP over IGP, and so on.