After the LDP activates an FEC for a prefix and programs RTM, it also programs the ingress tunnel table in IOM or on line cards with the LDP tunnel information.
When an IPv4 packet is received on an ingress network interface, a subscriber IES interface, or a regular IES interface, the lookup of the packet by the ingress IOM or line card results in the packet being sent labeled with the label stack corresponding to the NHLFE of the LDP LSP when the preferred RTM entry corresponds to an LDP shortcut.
If the preferred RTM entry corresponds to an IP next-hop, the IPv4 packet is forwarded unlabeled.
The switching from the LDP shortcut next-hop to the regular IP next-hop when the LDP FEC becomes unavailable depends on whether the next-hop is still available. If it is (for example, the LDP FEC was withdrawn because of LDP control plane issues) the switchover should be faster. If the next-hop determination requires IGP to re-converge, this takes longer. However, no target is set.
The switching from a regular IP next-hop to an LDP shortcut next-hop usually occurs only when both are available. However, the programming of the NHLFE by LDP and the programming of the LDP tunnel information in the ingress IOM or line cards tunnel table are asynchronous. If the tunnel table is configured first, it is possible that traffic is black-holed for some time.