IS-IS Multi-Instance
obsoletes
- rfc-6822 — IS-IS Multi-Instance
Extracted elements (25)
Dedicated multicast MAC addresses (AllL1MI-ISs, AllL2MI-ISs) are used for non-zero IID PDUs on broadcast circuits so that legacy routers silently ignore these PDUs rather than misinterpreting them as standard instance traffic, preventing LSDB corruption and incorrect adjacency/DIS state in legacy systems.
ITID #0 is reserved to enable RFC 5120 Multi-Topology (MT) operation within a non-zero instance as an alternative to MI-topology. When ITID #0 is the sole ITID of a non-zero instance, standard M-ISIS MT behavior applies; this allows controlling L1 information leaking and supporting incongruent topologies for different address families.
Graceful restart extensions (RFC 5306) apply to MI-RTRs, with the addition that all topology-specific LSP databases must be synchronized following restart. This requires additional T2 timers for each topology-specific LSDB.
RFC 8202 relaxes the RFC 6822 prohibition on using RFC 5120 MT within a non-zero instance: MT per RFC 5120 is now permitted when ITID #0 is the sole ITID of the instance. This change is not backwards compatible with strict RFC 6822 implementations, though all known implementations are compatible.
There is no relationship between the 12-bit MTIDs defined in RFC 5120 (M-ISIS) and the 16-bit ITIDs defined in this document. When an MI-RTR uses ITIDs and also supports the BFD-Enabled TLV (RFC 6213), the ITID MUST be used in place of the MTID.
An MI-RTR MUST NOT support RFC 5120 multi-topology within a non-zero instance when any non-zero ITID is supported. TLVs 222 (MT IS Neighbors), 235 (MT IP Reachability), and 237 (MT IPv6 Reachability) MUST NOT be sent in LSPs associated with a non-zero instance supporting a non-zero ITID, and such LSPs MUST be ignored when received.
For non-zero instances, an adjacency SHOULD NOT be established over a point-to-point circuit unless there is at least one ITID in common between the two routers.
IIDs MUST be unique within the same routing domain. IID #0 is reserved for the standard instance supported by legacy systems, and IS-IS PDUs associated with the standard instance MUST NOT include an IID-TLV (except as noted).
ITID #0 MUST NOT be supported in combination with any non-zero ITID. If multiple ITIDs are advertised in an IIH and one is #0, the PDU MUST be ignored. When multiple ITIDs are supported by an instance, ITID #0 MUST NOT be supported.
MI-RTRs MUST discard IS-IS PDUs if: (a) the destination multicast address is AllL1IS, AllL2IS, or AllIS and the PDU contains an IID-TLV, or (b) the destination is AllL1MI-ISs or AllL2MI-ISs and the PDU contains an IID-TLV with IID=0 or has no IID-TLV.
Multiple IID-TLVs MAY appear in IIHs; if present and the IID value in all IID-TLVs is not the same, the PDU MUST be ignored. The supported ITID set is the union of all ITIDs across all IID-TLVs.
Once an IIH with no IID-TLV is received on a point-to-point circuit (indicating a non-MI-RTR neighbor), the MI-RTR MUST NOT send IIHs for a non-zero instance. Before receiving any IIH, an MI-RTR MAY send IIHs for a non-zero instance.
Point-to-point adjacency setup MUST use the three-way handshaking procedure (RFC 5303) to prevent a non-MI-capable neighbor from prematurely bringing up an adjacency upon receiving an IIH with an IID-TLV for a non-zero instance.
To interoperate with a non-MI-capable neighbor over a point-to-point circuit, an MI-RTR MUST NOT send IS-IS PDUs for instances other than IID #0 over that circuit. All IIHs sent on a point-to-point circuit by an MI-RTR MUST include an IID-TLV (including those for IID #0) to signal MI-RTR capability.
When an LSP purge is initiated, the IID-TLV MUST be retained, but the remainder of the body of the LSP SHOULD be removed, following the purge procedures in RFC 6233 and RFC 6232.
When operating in point-to-point mode on a broadcast circuit (RFC 5309), an MI-RTR MUST use AllL1MI-ISs or AllL2MI-ISs when sending SNPs, LSPs, and point-to-point IIHs for a non-zero IID.
When sending SNPs, LSPs, and LAN IIHs for any non-zero IID on a broadcast circuit, an MI-RTR MUST use one of two dedicated Layer 2 multicast addresses (AllL1MI-ISs or AllL2MI-ISs) as the destination address.
When the IID-TLV appears (with a non-zero IID) in an SNP or LSP, exactly one ITID MUST be present. If no ITIDs or multiple ITIDs are present, or the IID is zero, the PDU MUST be ignored.
For non-zero instances, a unique Update Process exists for each supported ITID, each operating on a topology-specific LSDB. On point-to-point circuits, the ITID-specific Update Process only operates for ITIDs supported by both ISs.
On multi-access (broadcast) circuits, MI-RTRs establish adjacencies and elect a Designated Intermediate System (DIS) per IS-IS instance independently. Different instances may elect different DIS routers, and each MI-RTR only forms adjacencies with routers advertising support for the same instances.
The Instance Identifier TLV (IID-TLV, Type 7) carries a 16-bit Instance Identifier (IID) and zero or more 16-bit Instance-specific Topology Identifiers (ITIDs). It is included in all IS-IS PDUs (IIHs, LSPs, SNPs) originated by MI-RTRs for non-zero instances. A PDU without an IID-TLV belongs to the standard instance (IID #0).
IANA registered IS-IS TLV Type 7 ('Instance Identifier') in the IS-IS TLV Codepoints Registry, with allowed use in IIH, LSP, SNP, and Purge PDUs.
IANA registered two EUI-48 multicast addresses in the IANA Multicast 48-bit MAC Addresses registry: 01-00-5E-90-00-02 (AllL1MI-ISs) and 01-00-5E-90-00-03 (AllL2MI-ISs), for use by MI-RTRs sending Level-1 and Level-2 IS-IS PDUs with non-zero IIDs.
Security concerns for IS-IS are addressed in ISO 10589, RFC 5304 (IS-IS Cryptographic Authentication), and RFC 5310 (IS-IS Generic Cryptographic Authentication). When authentication is in use, the IID is first used to select the applicable authentication configuration before the authentication check is performed.
The IID-TLV has Type=7, Length=2-254. The Value field begins with a 2-octet IID (0-65535), followed by zero or more 2-octet Supported ITID fields. A single IID-TLV supports advertisement of up to 126 ITIDs.