The procedures at the iLER are as specified in section 4.2 of RFC 6790. In general, the router inserts an entropy label into the label stack if the downstream node for the LSP tunnel has signaled support for entropy label and the entropy label is enabled for the particular service.
RFC 6790 specifies that the iLER can insert several entropy labels in the label stack where the LSP hierarchy exists, one for each LSP in the hierarchy. However, this could result in unreasonably large label stacks. Therefore, when there are multiple LSPs in a hierarchy (for example, LDP over RSVP-TE), the router only inserts a single EL/ELI pair within the innermost LSP label closest to the service payload that has advertised entropy label capability.
The router inserts an entropy label on a tunnel that is entropy label-capable when the service has entropy label enabled, even if an implicit or explicit NULL label has been signaled by the downstream LSR or LER. This ensures consistent behavior and ensures that the entropy label value as determined by the iLER is maintained where a tunnel with an implicit NULL label is stitched at a downstream LSR.