IS-IS Routing with Reverse Metric
Extracted elements (24)
For LDP/IGP synchronization on LANs, Reverse Metric provides a simpler alternative to RFC 6138's approach: a new node advertises maximum inbound metric via Reverse Metric TLV, making only its own link last-resort without requiring all LAN nodes to change their metrics.
The metric value in the Reverse Metric TLV is defined as an offset (delta) to be added to the existing configured metric rather than an absolute value. This preserves existing operator provisioning and allows incremental adjustment without overwriting local configuration.
The Reverse Metric is carried in IIH PDUs rather than LSPs so that provisioning needs to be performed only on a single node, allowing traffic to shift bidirectionally away from a link without reconfiguring the remote neighbor. This avoids the operational complexity of coordinating metric changes on both ends simultaneously.
The Reverse Metric TLV is applicable to Multi-topology IS-IS (M-ISIS, RFC 5120). On point-to-point links, a received Metric value must be added to the Default Metric for all topologies in all Extended IS Reachability TLVs, ensuring consistent metric adjustment across all MT instances.
A sender MUST transmit only a single Reverse Metric TLV in an IS-IS Hello PDU. If a received IIH contains more than one Reverse Metric TLV, an implementation MUST ignore all the Reverse Metric TLVs.
For LDP/IGP synchronization on LANs, a new node SHOULD advertise the maximum metric offset value in the Reverse Metric TLV in its IIH PDUs until all LDP label binding exchanges complete or the provisioned timeout is exceeded.
If an implementation enables the Reverse Metric mechanism by default, it is RECOMMENDED that operators disable it when not explicitly using it.
If an IS-IS router is configured to originate a Traffic Engineering Default Metric sub-TLV for a link but receives a Reverse Metric TLV without a TE Default Metric sub-TLV, the router MUST NOT change the value of its TE Default Metric sub-TLV for that link.
If the sub-TLV Len field is non-zero, the Value field MUST also contain one or more sub-TLVs. The sub-TLV Len value MUST be set to zero when an IS-IS router does not have Traffic Engineering sub-TLVs to send.
Local provisioning on the DIS to adjust Default Metrics MUST take precedence over received Reverse Metric TLVs when both are present.
On a multi-access LAN, only the DIS SHOULD act upon a received Reverse Metric TLV. All non-DIS nodes MUST silently ignore a received Reverse Metric TLV.
On point-to-point links, an M-ISIS router MUST send only a single Reverse Metric TLV in IIH PDUs. When received, it MUST add the Metric value to the Default Metric for all topologies in all Extended IS Reachability TLVs.
On receiving a Reverse Metric TLV, the accumulated value of the configured metric plus the reverse-metric offset MUST be limited to 63 in narrow metric mode and to (2^24 - 2) in wide metric mode. If the U bit is set, the limit is (2^24 - 1) for both normal and TE metrics.
The W (Whole LAN) bit MUST be clear when a Reverse Metric TLV is transmitted in an IIH PDU on a point-to-point link and MUST be ignored when received on a point-to-point link.
When multiple non-DIS nodes send Reverse Metric TLVs with the W bit set, the DIS MUST use the reverse metric of the highest source MAC address non-DIS advertising the Reverse Metric TLV with the W bit set.
When the link Traffic Engineering metric is raised to (2^24 - 1), this SHOULD immediately trigger CSPF recalculation to move TE traffic away from that link. It is RECOMMENDED that CSPF also immediately recalculate when TE metric is raised to (2^24 - 2) as a last resort link.
The Metric field in the Reverse Metric TLV is a 24-bit unsigned integer offset in the range 0–63 for narrow IS-IS metric mode, or 0–(2^24 - 2) for wide metric mode. The accumulated wide metric value with U bit set is bounded at (2^24 - 1), which represents non-reachability; (2^24 - 2) is the last-resort link metric.
The Reverse Metric TLV (Type 16) is a new TLV carried inside IS-IS Hello (IIH) PDUs. It contains a 1-octet Flags field, a 3-octet Metric field (24-bit unsigned integer offset), a 1-octet sub-TLV length, and optional sub-TLVs. Total length is variable (5–255 octets).
The Traffic Engineering Metric sub-TLV (Type 18) is an optional sub-TLV of the Reverse Metric TLV containing a 24-bit unsigned integer TE metric offset. If it appears more than once, the entire Reverse Metric TLV MUST be ignored.
The U bit (0x02) in the Flags field specifies that the accumulated metric (reverse metric plus default metric) is limited to the maximum value of (2^24 - 1), marking the link as unreachable. This applies only in IS-IS wide metric mode.
The W bit (0x01) in the Flags field is used only on multi-access LANs. When set and sent from a non-DIS to the DIS, it signals the DIS to add the received Metric value to all nodes' Default Metrics in the Pseudonode LSP. When clear, the DIS adjusts only the originating node's metric.
IANA has allocated IS-IS TLV Codepoint 16 for the Reverse Metric TLV with attributes: IIH=y, LSP=n, SNP=n, Purge=n. A new sub-TLV registry 'Sub-TLVs for TLV 16 (Reverse Metric TLV)' is created under the IS-IS TLV Codepoints registry with Expert Review policy. Sub-TLV type 18 is assigned for the Traffic Engineering Metric.
The Reverse Metric mechanism enables one IS-IS router to manipulate the Default Metric and optionally Traffic Engineering parameters of adjacent IS-IS neighbors. Operators are highly recommended to configure IS-IS PDU authentication (per RFC 5304/RFC 5310) to mitigate use of the Reverse Metric TLV as a potential attack vector.
The Reverse Metric TLV wire format: Type (1 octet) = 16, Length (1 octet, variable 5–255), Flags (1 octet), Metric (3 octets, 24-bit unsigned integer), sub-TLV Len (1 octet), sub-TLV data (0–250 octets). The Flags field has two defined bits: W (0x01, Whole LAN) and U (0x02, Unreachable); reserved bits MUST be zero.