Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS
obsoleted by
- rfc-7176 — Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS
Extracted elements (31)
The Sender Nickname field in the VLAN-Flags sub-TLV is included to support intelligent end stations that determine the egress RBridge for unicast data through a directory service and need a first-hop nickname to correctly format TRILL-encapsulated frames, as described in RFC 6325 Section 4.6.2.
TRILL uses a fixed zero Area Address (Section 4.2), departing from standard IS-IS practice, because TRILL operates as a single flat domain where area-based hierarchy is neither required nor desired.
If the TRILL-VER sub-TLV is absent from an IS's Router Capability TLV, the originating IS is assumed to support only the base version of TRILL (RFC 6325). The sub-TLV's presence signals support for higher versions, with lower versions implicitly supported.
The Topology-ID field in the GMAC-ADDR sub-TLV is not used by TRILL — it is sent as zero and ignored on receipt — but is included in the format for use by other technologies that may reuse this sub-TLV structure.
An INT-VLAN sub-TLV MUST be split into subranges whenever the information (multicast router attachment, appointed forwarder status lost counter, root bridges) differs across VLANs in the range. Using a range of one VLAN ID is always safe.
An IS receiving an update to an INT-VLAN sub-TLV SHOULD employ a hold-down timer before processing it, to allow multiple LSP fragments of the same update to arrive before processing begins, minimizing disruption.
In a TRILL IIH PDU, the Maximum Area Addresses octet in the IS-IS common header MUST be set to 0x01.
In the set of LSPs originated by an IS, the union of VLAN ranges in all INT-VLAN sub-TLV occurrences MUST be precisely the set of VLANs for which the IS is appointed forwarder on at least one port, and VLAN ranges in multiple INT-VLAN sub-TLVs for an IS MUST NOT overlap unless the per-VLAN information is identical.
The IS-IS Neighbor TLV (type 6) MUST NOT be used in a TRILL IIH PDU; if present it is ignored. TRILL IIH PDUs use the TRILL Neighbor TLV (type 145) instead.
The Sender Nickname field in the VLAN-Flags sub-TLV MUST include one of the IS's nicknames if it holds any; otherwise the field is set to zero.
TRILL uses a fixed zero Area Address encoded as a 4-byte Area Address TLV (type 1). The Protocols Supported TLV (type 129) including NLPID 0xC0 MUST appear in TRILL IIH PDUs and LSP number zero PDUs.
When a tree identifier can be derived from two conflicting TREE-RT-IDs sub-TLVs, that tree SHOULD NOT be computed unless computation is indicated by all relevant sub-TLVs, treating the discrepancy as a transient condition.
When MAC addresses in the TRILL Neighbor TLV span multiple TLVs or PDUs, the highest MAC address in a TLV with L=0 MUST also appear in another TRILL Neighbor TLV, and the lowest MAC address in a TLV with S=0 MUST also appear in another TRILL Neighbor TLV. An RBridge with no neighbors MUST send a TRILL Neighbor TLV with an empty record list (both S and L set).
The Group Address (GADDR) TLV, IS-IS TLV type 142, is carried only in LSP PDUs and contains sub-TLVs advertising multicast group listeners. Its sub-TLVs form a new sub-TLV series distinct from other IS-IS sub-TLV sequences.
A new IANA sub-registry 'IS-IS Group Address Type Codes for TLV 10' is created for sub-TLVs of the GADDR TLV, with Expert Review registration procedure. Initial entries: value 0 (Reserved), value 1 (GMAC-ADDR), values 2–254 (Unassigned), value 255 (Reserved).
Two new IS-IS PDU types are registered: MTU-PROBE-PDU (type 23) and MTU-ACK-PDU (type 28), along with a new IS-IS PDU registry (Appendix A) covering the full PDU type space with IETF Review registration procedure.
Two new TLV types are registered in the IANA IS-IS TLV registry: GADDR-TLV (type 142, LSP only, multiple occurrences allowed) and TRILL Neighbor TLV (type 145, IIH only, multiple occurrences allowed). Eleven new sub-TLVs are registered across the MT-PORT-CAP, Router Capability, and Extended IS Reachability TLV sub-registries.
IS-IS authentication per RFC 5304 and RFC 5310 may be used to secure the IS-IS messages defined here. However, even with authentication, replays of Hello packets can create denial-of-service conditions (see RFC 6039); these issues are analogous to those in RFC 6325 Section 6.2 and the same mitigations apply.
The Appointed Forwarders sub-TLV (MT-PORT-CAP sub-TLV type 3, length 6*n) is sent by the DRB and contains n appointment records each with a 2-byte Appointee Nickname and two 2-byte fields for Start.VLAN and End.VLAN specifying the inclusive VLAN range for which the IS is appointed forwarder.
The Enabled-VLANs sub-TLV (MT-PORT-CAP sub-TLV type 2, minimum length 3) advertises VLANs enabled for end-station service at the sending port. It encodes a 12-bit Start VLAN ID followed by a bit-map where each bit represents consecutive VLAN IDs starting from Start VLAN ID.
The Group MAC Address (GMAC-ADDR) sub-TLV (type 1 within GADDR TLV) encodes multicast listeners by 48-bit group and source MAC address. Fields include a 2-byte Topology-ID (unused in TRILL, sent zero), a 2-byte VLAN ID, a 1-byte group record count, and variable-length group records each comprising a 1-byte source count, a 6-byte group address, and zero or more 6-byte source addresses.
The Interested VLANs (INT-VLAN) sub-TLV (Router Capability sub-TLV type 10, length 10+6*n) encodes a 2-byte Nickname, a 4-byte Interested VLANs field with M4/M6 multicast flags and start/end VLAN IDs, a 4-byte Appointed Forwarder Status Lost Counter, and n 6-byte root bridge IDs.
The MTU sub-TLV (Extended Reachability TLV sub-TLV type 28, length 3) contains a 1-byte field with an F (Failed) flag, followed by a 2-byte MTU field set to the largest successfully tested MTU for the link, or zero if untested.
The Nickname sub-TLV (Router Capability sub-TLV type 6, length 5*N) carries one or more nickname records, each comprising a 1-byte Nickname.Pri (priority to hold the nickname), a 2-byte Tree Root Priority, and a 2-byte Nickname value.
The Special VLANs and Flags (VLAN-Flags) sub-TLV (MT-PORT-CAP sub-TLV type 1, length 8) is carried in every TRILL IIH PDU and contains a 2-byte Port ID, a 2-byte Sender Nickname, a 2-byte field with flags AF/AC/VM/BY and 12-bit Outer.VLAN, and a 2-byte field with flag TR and 12-bit Desig.VLAN.
The TREE-RT-IDs sub-TLV (Router Capability sub-TLV type 8, length 2+2*n) provides an ordered list of tree root nicknames with a 2-byte Starting Tree Number to allow correct concatenation when the list is split across multiple sub-TLV instances.
The TREES sub-TLV (Router Capability sub-TLV type 7, length 6) contains three 2-byte unsigned integers: number of trees to compute, maximum trees able to compute, and number of trees to use, supporting distribution tree computation.
The TRILL Neighbor TLV (type 145, length 1+9*n) replaces the IS Neighbor TLV in TRILL IIH PDUs. It has a 1-byte header with S (Smallest) and L (Largest) flags, followed by n neighbor records each containing a 1-byte F (failed MTU) flag, a 2-byte MTU, and a 6-byte MAC address.
The TRILL Version (TRILL-VER) sub-TLV (Router Capability sub-TLV type 13, length 1) contains a single byte Max-version field indicating the highest TRILL standard version supported; lower versions are implicitly supported as well.
The VLAN Group sub-TLV (Router Capability sub-TLV type 14, length 4+2*n) carries a 2-byte Primary VLAN ID and one or more 2-byte Secondary VLAN IDs (each with a 4-bit RESV prefix), indicating shared VLAN learning at the announcing IS.
Two new IS-IS PDUs are defined: MTU-PROBE-PDU (type 23) and MTU-ACK-PDU (type 28). Each carries a 20-byte fixed header comprising 2-byte PDU Length, 6-byte Probe ID, 6-byte Probe Source ID, and 6-byte Ack Source ID; the Ack Source ID is zero in probes and set to the responder's System ID in acks.