IS-IS Path Control and Reservation
Extracted elements (31)
IS-IS PCR only addresses point-to-point physical links (not shared media LANs) because it relies on the SPB architecture where parallel links between adjacent bridges are distinguished by the Extended Local Circuit ID in the Hop sub-TLV.
The GADAG is identical for all MRTs within a network domain (a consequence of the MRT Lowpoint algorithm), making it beneficial to compute the GADAG by a single GADAG Computer entity rather than distributedly. The MRTG ECT Algorithm exploits this to centralize GADAG computation and distribute results via IS-IS flooding.
The Timestamp sub-TLV uses TAI (International Atomic Time) seconds from the PTP epoch, explicitly excluding leap seconds, in order to align with IEEE 1588 Precision Time Protocol semantics and enable consistent precedence ordering across PCEs.
IS-IS PCR does not require implementation of the full IS-IS SPB protocol, but only support of specific SPB sub-TLVs (SPB Link Metric, SPB Base VLAN-Identifiers, SPB Instance, SPBV MAC address, SPBM Service Identifier and Unicast Address). A bridge can support both IS-IS SPB and IS-IS PCR simultaneously via the same IS-IS entity on a per-instance basis.
IS-IS PCR does not use the MRT Profile specified in RFC 7812; it only relies on the algorithm specification in RFC 7811 (MRT Lowpoint algorithm). Both the MRT ECT Algorithm and the MRTG ECT Algorithm use this algorithm.
When the Unreserved Bandwidth sub-TLV is used in a Layer 2 bridge network, the priority value encoded provides the PCP (traffic class identifier), not a setup priority level as in IP/MPLS TE contexts.
An explicit tree MUST be described by the Topology sub-TLV, and the Topology sub-TLV MUST be carried in an MT-Capability TLV (type 144) in a Link State PDU.
An Explicit Tree MUST NOT contain cycles. It is the responsibility of the PCE to ensure reverse-path congruency and multicast-unicast congruency if required.
Both MSRP and IS-IS MUST NOT be used to make bandwidth assignments in the same domain; only one reservation mechanism may operate at a time.
Each Edge Bridge of an explicit tree MUST always be specified in the Topology sub-TLV by inclusion of the corresponding Hop sub-TLVs with the Edge Bridge (B) flag set.
If a PCE is not part of the IS-IS domain, then the PCE MUST be associated with a PCA that is part of the IS-IS domain, and the PCE or its PCA MUST establish IS-IS adjacency to receive all LSPs.
If bandwidth assignment would result in overbooking any link of the explicit tree, then bandwidth assignment MUST NOT be performed and a management report is generated.
If the MRTG ECT Algorithm is applied, then the GADAG MUST be computed only by the GADAG Computer, which then MUST flood the descriptor Topology sub-TLV of the GADAG.
If the SPB Link Metric values advertised by different ends of an adjacency are different, then the maximum value MUST be used for IS-IS PCR including MRT algorithms.
The bridge with the best BridgeID (numerically lower is better) MUST be selected as the GADAG Root; Bridge Priority in the SPB Instance sub-TLV allows management configuration of the GADAG Root.
The Hop sub-TLV MUST be used to specify a hop of a topology; the Edge Bridge flag MUST be set if the system is an Edge Bridge, and the Root flag MUST be set if the system is a root of the explicit tree.
The path of a loose hop MUST be pruned from the tree if the path would create a loop or hairpin.
The Strict Tree (ST) ECT Algorithm (value 00-80-C2-17) MUST be used for a strict explicit tree; a strict ET is static and only the owner PCE can update it.
When using the LT or LTS ECT Algorithms, if equal cost (sub-)paths are found during MRT computation, the tie-breaking specified by Section 11 of RFC 6329 MUST be used, based on the lower BridgeID.
A GADAG is encoded in a Topology sub-TLV using directed ear decomposition: each ear is an ordered list of hops per the direction of GADAG arcs; the Leaf flag marks end of a topology block; the first hop is always the GADAG Root; MRT Roots are marked with the Root flag.
Bandwidth assignment precedence ordering is determined by: (1) PCP parameter (higher wins), (2) Importance parameter within PCP (lower numerical value wins), (3) Timestamp sub-TLV if present (earlier wins). If unresolvable, the PCE with the numerically lowest LSP ID is granted precedence.
Five explicit ECT Algorithms are defined: Strict Tree (ST, 00-80-C2-17), Loose Tree (LT, 00-80-C2-21..30), Loose Tree Set (LTS, 00-80-C2-31..40), MRT (00-80-C2-18), and MRT GADAG (MRTG, 00-80-C2-19). Each uses the IEEE 802.1 OUI concatenated with an index.
The Hop sub-TLV (type 22) specifies a single hop of a topology using an IS-IS System ID (48 bits). It conveys a flags byte (C, V, B, R, L, E flags), the System ID, an optional Extended Local Circuit ID (32 bits), optional VIDs with T/R flags, and an optional 48-bit Delay Constraint.
The Topology sub-TLV (type 21) is the primary sub-TLV for describing an explicit tree or GADAG. It MUST be carried in an MT-Capability TLV (type 144) in a Link State PDU, and contains a count of Base VIDs (12-bit each), followed by variable-length sub-TLVs specifying hops and constraints.
Five new sub-TLVs are registered within the MT-Capability TLV (type 144) in the IS-IS TLV Codepoints registry: type 21 (Topology, variable length), type 22 (Hop, variable length), type 23 (Bandwidth Constraint, 5 bytes), type 24 (Bandwidth Assignment, 5 bytes), type 25 (Timestamp, 4 bytes).
IS-IS PCR adds no additional security risks to IS-IS but is not suitable for zero-configuration environments. Any path selection and resource allocation mechanism is potentially vulnerable to attack; the security considerations of RFC 4655 (PCE architecture) apply. IS-IS routing security measures including peer advertisement authentication via RFC 5310 should be applied.
Bandwidth Assignment sub-TLV format: Type (8 bits, value 24), Length (8 bits, value 5), PCP (3 bits), DEI/D flag (1 bit), Importance (3 bits), Reserved (1 bit), Bandwidth (32 bits as IEEE 754 floating-point, units bytes/second).
Bandwidth Constraint sub-TLV format: Type (8 bits, value 23), Length (8 bits, value 5), PCP (4 bits), DEI/D flag (1 bit), P flag (1 bit), Reserved (3 bits), Available Bandwidth (32 bits as IEEE 754 floating-point, units bytes/second).
Hop sub-TLV format: Type (8 bits, value 22), Length (8 bits), Hop Flags (8 bits: C|V|B|R|L|E|Res), System ID (48 bits), optional Extended Local Circuit ID (32 bits), optional Number of VIDs (8 bits) with VID entries (T|R|Res|VID, 14 bits each), optional Delay Constraint (48 bits).
Timestamp sub-TLV format: Type (8 bits, value 25), Length (8 bits, value 4), Time (32 bits) representing seconds since the PTP epoch (1970-01-01 00:00:00 TAI, no leap seconds).
Topology sub-TLV format: Type (8 bits, value 21), Length (8 bits), Number of Base VIDs (8 bits), followed by Base VID entries (4-bit Reserved + 12-bit VID, 2 bytes each), then variable-length sub-TLVs for hops and constraints.