The SR OS router follows the procedures at the ingress LER as specified in Section 4.2 of RFC 6790. In general, the router inserts an EL in a packet if the egress LER for the LSP tunnel has signaled support for ELs, the EL is configured for the service that the packet belongs to, and the EL is not disabled for an RSVP LSP. If there are multiple LSPs in a hierarchy (for example, LDP over RSVP), the router only inserts a single EL and ELI pair under the innermost LSP label closest to the service payload that has advertised EL capability. The router does not insert an EL in a packet belonging to a service for which the hash label has been configured, even if the far end for the LSP tunnel has advertised ELC. The system instead inserts a hash label, as specified by the hash label feature.
If the downstream LSR or LER has signaled implicit or explicit NULL label for a tunnel that is ELC, the router still inserts the EL when required by the service. This ensures consistent behavior as well as ensuring that entropy as determined by the ingress LER is maintained where a tunnel with an implicit NULL label is stitched at a downstream LSR.