IP to MPLS
    ip leaf > ldi > li > nh

MPLS to MPLS
    mpls leaf > ldi > li > nh
    mpls leaf > ldi > imp_null_li1 > nh

MPLS to IP
    mpls leaf > ldi > unlabelled_lable_li > nh (v4)
    mpls leaf > ldi > imp_null_li > nh (v4)

MPLS VPN Imp
    vrf_leaf > ldi > vpn_li > ldi > igp_li > nh

MPLS VPN Disp
    vpn_label_entry > ldi > imp-null-li > nh (v4)

VPN de-aggregate
    vpn_label_entry > ldi > deagg_li

TLU2 labelA: IGP or TE-midpoint label
TLU3 labelB: TE headend label
TLU3 labelC: merge point label (active / temporary / "FRR" / uIDB idx)
TLU4 labelD: TE-FRR (headend label of backup tx adj)

Temporary TE adj entry is used during TE reopt
uIDB idx corresponding to backup interface

Backwalk
when tunnel is modified to protect tunnel (because te-protect field is in TLU2)
when tunnel re-opt is going out on different interface (because uidb idx is in TLU2)
when destination mac address of interface corresponding to tunnel changes (bcause in TLU2)

Question : when multiple backup, just pick one ? or loadbalance ?