A YANG Module for IS-IS Reverse Metric
Extracted elements (16)
Level-specific reverse metric configuration (level-1, level-2 containers) is only available for broadcast interface types because the reverse metric is advertised in IS-IS Hello PDUs (IIH PDUs), which carry level information only on broadcast links.
The YANG module uses the same per-level hierarchical configuration structure as defined in the augmented base IS-IS module (RFC 9130), maintaining consistency with existing IS-IS YANG conventions.
The ietf-isis-reverse-metric module imports from ietf-routing (RFC 8349) and ietf-isis (RFC 9130), and the augment when-conditions ensure the augmentations apply only when the routing protocol instance type is 'isis:isis'.
By default, reverse metric handling is disabled and must be explicitly enabled via the enable-receive configuration leaf; without this, a router ignores reverse metric announcements from neighbors.
Reverse metric operation on an interface is enabled by configuring a metric value either at the top level or under a level-specific container. If a metric value is only specified under a level-specific container, operation is only enabled at that level.
The allow-unreachable leaf (U bit from RFC 8500) allows the neighbor to increment the overall metric up to 2^24-1 rather than the lesser maximum of 2^24-2; a metric of 2^24-1 causes traffic to stop using the interface entirely.
The exclude-te-metric leaf (default false) controls whether the Traffic Engineering Metric sub-TLV is suppressed in the Reverse Metric TLV when a TE metric is defined for the interface.
The ietf-isis-reverse-metric YANG module augments the IS-IS instance with a global reverse-metric container containing an enable-receive leaf (boolean, default false) that controls whether reverse metric announcements from neighbors are processed.
The module augments each IS-IS interface with a reverse-metric container for announcing reverse metrics to neighbors, including metric (wide-metric type), flags (whole-lan and allow-unreachable booleans), and exclude-te-metric leaf. Level-specific sub-containers (level-1, level-2) are available only for broadcast interface types.
The module augments IS-IS adjacency state with a read-only reverse-metric container exposing the received Reverse Metric TLV data: metric (wide-metric), flags (whole-lan, allow-unreachable), and te-metric (uint32 from the sub-TLV if present).
The whole-lan leaf (W bit from RFC 8500) controls whether a DIS processing the reverse metric adds the metric value to all nodes it advertises in the pseudonode LSP, or only to the advertising node.
This document registers the URI urn:ietf:params:xml:ns:yang:ietf-isis-reverse-metric in the IETF XML Registry (RFC 3688) as an XML namespace with the IESG as registrant contact.
This document registers the YANG module 'ietf-isis-reverse-metric' in the YANG Module Names registry (RFC 6020) with prefix isis-rmetric, namespace urn:ietf:params:xml:ns:yang:ietf-isis-reverse-metric, not maintained by IANA.
Read-only adjacency state nodes (metric, whole-lan, allow-unreachable, te-metric under the adjacency reverse-metric container) may be sensitive, requiring controlled read access via NETCONF get/get-config or notifications.
The YANG module is accessed via NETCONF or RESTCONF; NETCONF requires SSH as the mandatory secure transport, and RESTCONF requires TLS 1.3. NACM (RFC 8341) provides access control for restricting operations and content to authorized users.
Writable data nodes (enable-receive, metric, whole-lan, allow-unreachable, exclude-te-metric at interface and per-level scopes) are sensitive because unauthorized write operations can alter routing metrics and negatively affect network operations; security implications are described in RFC 8500.