The MPLS entropy label provides a similar function to the hash label, but is applicable to a wider range of services. The entropy label is appended directly below the tunnel label. As with the hash label, the value of the entropy label is calculated based on a hash of the packet payload header.
The router supports the entropy label for the following services and protocols:
VPRN
EVPN VPLS and Epipe
RFC 3107 MP-BGP tunnels
RSVP and LDP LSPs used as shortcuts for static, IGP, and BGP route resolution
VLLs, including BGP VPWS, IES/VPRN, and VPLS spoke-SDP termination, but not including Cpipe
LDP VPLS and BGP-AD VPLS
PW ports bound to a specific physical port supporting PW-SAPs used for Epipe VLL, IES, VPRN, and Enhanced Subscriber Management services
The entropy label is supported with the following tunnel types:
RSVP-TE: configured and auto-LSPs
LDP
Segment Routing (shortest path, PCC and PCE-initiated SR-TE and SR-TE auto-LSPs)
BGP
The entropy label is not supported on P2MP LSPs.
The entropy label indicated (ELI) label (value=7) is a special-purpose label that indicates that the entropy label follows in the stack. It is always placed immediately below the tunnel label to which hashing applies. Inserting the EL adds two labels in the MPLS label stack: the EL and its accompanying ELI.
Three criteria are used to determine if an EL and an ELI are inserted on a labeled packet belonging to a service by an ingress LER:
the Entropy Label Capability (ELC)
ELC is the ability of the egress LER to receive and process the EL. The ingress LER associates the ELC with the LSP tunnel to be used to transport the service. ELC signaling is supported for RSVP and LDP and causes the router to signal ELC to upstream peers. ELC is configured on these services by using the config>router>rsvp>entropy-label-capability and config>router>ldp>entropy-label-capability commands.
ELC signaling is not supported for BGP or SR tunnels. For these services, configure the ingress LER (or LSR at a stitching point to a BGP or SR segment) with ELC for this tunnel type using the override-tunnel-elc command for BGP or for the IGP if using SR.
whether a specific tunnel at the ingress LER supports EL
Support for EL on a specific tunnel is configurable to prevent exceeding the maximum supported label stack depth because of the additional EL and ELI label (see Impact of EL and ELI on MTU and label stack depth for more information). For RSVP and SR-TE LSPs, it is configured using the entropy-label command under the LSP, LSP template, or MPLS contexts.
whether the use of EL has been configured for the service
See the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide: VLL, VPLS, PBB, and EVPN, 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide: IES and VPRN, and the 7450 ESS, 7750 SR, 7950 XRS, and VSR Unicast Routing Protocols Guide for more information about entropy label configuration on services.
Each of these conditions must be true before the ingress LER inserts the EL and ELI into the label stack.
An LSR for RSVP and LDP tunnels passes the ELC from the downstream LSP segment to upstream peers. However, releases of SR OS that do not support EL functionality do not pass the ELC to their peers.