Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS
Extracted elements (17)
ELC is associated and advertised per-prefix rather than solely per-node because in multi-area or multi-domain networks, ingress LSRs may not know the identity or capabilities of a prefix originator in a remote area or domain.
ERLD is useful in SR-MPLS deployments so that ingress LSRs can determine the position of the EL label in the stack and whether it is necessary to insert multiple ELs at different positions.
The absence of ERLD-MSD advertisements indicates only that the advertising node does not support advertisement of this capability, not that the node lacks the capability itself.
The exact mechanism to exchange ELC between protocol instances running on an Autonomous System Border Router during redistribution is outside the scope of this document.
The IS-IS ELC and ERLD extensions defined in this document can be advertised via BGP-LS using existing TLVs: ELC via the Prefix Attribute Flags TLV (RFC 9085) and ERLD-MSD via the Node MSD TLV (RFC 8814).
A router MUST NOT announce the ELC for any local host prefixes unless all of its interfaces are capable of processing Entropy Labels.
If a router has multiple interfaces with different capabilities for reading the maximum label stack depth, the router MUST advertise the smallest ERLD value found across all its interfaces.
If a router supports ELs on all of its interfaces, it SHOULD set the ELC for every local host prefix it advertises in IS-IS.
If the ERLD-MSD type is received in the Link MSD sub-TLV, it MUST be ignored.
The ELC signaling MUST be preserved when a router propagates a prefix between IS-IS levels (per RFC 5302).
When redistributing a prefix between two IS-IS protocol instances or from another protocol to IS-IS, a router SHOULD preserve the ELC signaling for that prefix if it exists.
ERLD-MSD is a new MSD-Type (code 2 in the IGP MSD-Types registry) used to advertise the Entropy Readable Label Depth of a router. The MSD-Value field carries the ERLD in the range 0 to 255.
The ELC Flag (E-Flag) is defined as Bit 3 in the Prefix Attribute Flags sub-TLV (RFC 7794). It is set for a local host prefix of the originating node if that node supports ELC on all interfaces.
IANA assigned Bit 3 in the 'Bit Values for Prefix Attribute Flags Sub-TLV' registry to the ELC Flag (E-Flag), and Type 2 in the 'IGP MSD-Types' registry to ERLD-MSD.
Incorrectly setting the E-Flag during origination, propagation, or redistribution may lead to poor or no load-balancing of MPLS traffic, or to MPLS traffic being discarded on the egress node. Incorrectly setting the ERLD value may similarly lead to poor or no load-balancing.
The security considerations of RFC 7752, RFC 7794, RFC 7981, RFC 8491, RFC 8662, RFC 8814, and RFC 9085 are all applicable, as this document extends those protocols to advertise additional node capabilities via IS-IS and BGP-LS.
The Prefix Attribute Flags sub-TLV bit layout assigns bit 0 (X), bit 1 (R), bit 2 (N), and bit 3 (E/ELC Flag). The E-Flag occupies a single bit in the flags octet.