Transparent Interconnection of Lots of Links (TRILL): Fault Management
updates
- rfc-6325 — Routing Bridges (RBridges): Base Protocol Specification
Extracted elements (32)
A new Path Trace Message is defined rather than reusing the 802.1Q Link Trace Message because in TRILL, unicast and multicast paths are not congruent. Link Trace Messages use a well-known multicast MAC address, which follows the distribution tree rather than the unicast forwarding path, making them unsuitable for TRILL unicast fault isolation.
The hop-count-expiry (traceroute-like) approach is used for path tracing rather than explicit MIP addressing because any MIP within an ingress interface may reply without loss of functionality; the responding RBridge is identified from the Sender ID TLV (Chassis ID sub-type 16396) combined with the Reply Ingress TLV.
The TRILL OAM Message Channel reuses the 802.1Q CFM message format (Ethertype 0x8902) to promote a common OAM framework across technologies, enable nested fault monitoring, and allow direct reuse of CCM and LBM messages. Y.1731 adopts the same stance, making the approach broadly applicable.
CCM (Continuity Check Message) as defined in 802.1Q can be used in TRILL without modification because the MAID (48 bytes) and MEP-ID (2 bytes) fields are technology-independent; TRILL-specific information is carried via optional TLVs appended to the standard CCM format.
Intermediate RBridges that do not support the 'A' flag (OAM incapable) follow RFC 6325 Section 3.3 and forward OAM frames with the 'A' flag unaltered, enabling end-to-end OAM connectivity even in mixed-capability TRILL networks.
Destination RBridges receiving a Loopback Message MUST echo the received Loopback Transaction Identifier without modification. At each retransmission, the Session Identification Number MUST be incremented.
For the Authentication TLV, Auth Types 0, 1, 2, and 54 MUST NOT be used; Auth Type 3 (HMAC-based per RFC 5310, using the same Key ID space as TRILL IS-IS) is RECOMMENDED. Key distribution is out of scope and reuses IS-IS key distribution.
Frames with the 'A' flag set that do not contain a CFM Ethertype (0x8902) MUST be silently discarded. OAM-capable RBridges MUST NOT generate OAM frames to an RBridge that is not OAM capable.
Implementations complying with this document MUST utilize both the 'A' flag and CFM Ethertype 0x8902 together to identify TRILL OAM frames; neither condition alone is sufficient.
In TRILL, a MEP-ID MUST be a number in the range 1 to 65535, extending the 802.1Q CFM restriction of 1–8191 to enable 1-to-1 mapping with 16-bit RBridge nicknames used in the Base Mode.
On edge interfaces, MEPs MUST be configured to prevent leaking TRILL OAM packets out of the TRILL campus as native frames. The TRILL Encapsulation/Decapsulation layer is responsible for this containment.
RBridges MUST NOT use the 'A' Alert flag for forwarding decisions such as selection of which ECMP path or multi-destination tree to use.
The TRILL OAM Application Identifier TLV MUST always be present and MUST be the first TLV in every TRILL OAM message. Any TRILL OAM message that does not include this TLV as the first TLV MUST be discarded by a TRILL MP.
Four TRILL-specific CFM OpCodes are defined: 64 (Path Trace Reply), 65 (Path Trace Message), 66 (Multi-destination Tree Verification Reply), 67 (Multi-destination Tree Verification Message). Loopback (OpCodes 2/3) and CCM reuse 802.1Q OpCodes without modification.
OAM capabilities are announced via O and B flags in the TRILL Version sub-TLV (TRILL-VER): O=1/B=0 signals new-style OAM; O=1/B=1 signals backwards-compatible OAM; O=0 means OAM incapable (B ignored). Absence of the TRILL-VER sub-TLV also means the RBridge is not OAM capable.
The Multi-destination Tree Verification Message (MTVM, OpCode 67) is copied to the CPU of every intermediate RBridge on the distribution tree being verified. Only RBridges listed in the RBridge Scope TLV (Type 68) send replies; absent scope means all RBridges respond. Each Scope TLV holds up to 255 nicknames; multiple instances MAY be chained.
The Path Trace Message (OpCode 65) uses a traceroute-like hop-count-expiry approach: the originator starts with Hop Count=1, records each intermediate RBridge response (Return Sub-code 2), and increments the Hop Count by 1 for each retransmission until a Valid Response from the destination is received, a timeout occurs, or a configured maximum Hop Count is reached.
The TRILL Header 'A' (Alert) flag is assigned to the reserved bit next to the Version field. A value of 1 indicates a possible OAM frame; definitive identification requires also matching CFM Ethertype 0x8902 at the OAM Ethertype position in the packet. This flag updates RFC 6325, which had no OAM identification mechanism.
TRILL OAM packets consist of six parts: Link Header (media-dependent), TRILL Header (6+ bytes), Flow Entropy (96 bytes fixed, padded with zeros), OAM Ethertype (0x8902), OAM Message Channel (variable, CFM-formatted), and Link Trailer. Flow Entropy starts with Inner.MacDA and steers OAM frames along the same path as data packets.
A new TRILL OAM Capability Flags sub-registry is established under TRILL-VER sub-TLV. Two flags are defined: O (bit 2, OAM capable) and B (bit 3, backwards-compatible OAM), located after the existing A (Affinity) and F (FGL-safe) flags in the Capability and Header Flags field.
A Return Codes and Sub-codes registry is established for TRILL OAM. Defined entries include Return Code 1 ('Reply') with Sub-code 0 ('Valid Response') and Sub-code 2 ('Intermediate RBridge'). Return Sub-code values MUST be interpreted within the context of their specific Return Code.
Four TRILL-specific entries are registered in the IEEE 802.1Q CFM OpCode space: 64 (Path Trace Reply), 65 (Path Trace Message), 66 (Multi-destination Tree Verification Reply), and 67 (Multi-destination Tree Verification Message).
IANA Address Family Number 16396 is allocated for the TRILL Nickname address family, enabling its use in the 802.1Q Sender ID TLV Chassis ID field (Chassis ID sub-type = 16396) to uniquely identify the sender RBridge nickname in OAM messages.
An RBridge supporting OAM authentication can operate in three modes: (1) neither generate nor check Authentication TLVs, (2) include Authentication TLVs in sent messages but ignore them on receipt, or (3) enforce full authentication on both sent and received OAM messages, discarding unauthenticated or unverified messages.
TRILL OAM messages must be protected using the Authentication TLV (Type 74) to prevent unauthorized injection or replay of OAM messages (e.g., spoofed CCM or LBM frames) that could disrupt network operation or be used for reconnaissance. An RBridge enforcing authentication discards any OAM message that lacks a valid Authentication TLV.
At a MEP, OAM frames pass through a three-stage demultiplexer: (1) Tap captures frames with Alert flag and CFM Ethertype; (2) MD-Level demultiplexer drops frames with MD-Level less than the MEP's configured level, passes matching levels to the OpCode demux, and forwards higher levels onward; (3) OpCode demultiplexer dispatches to CCM, LBM, LBR, PTM, PTR, MTVM, or MTVR processors, discarding unknown OpCodes.
All TRILL OAM TLVs share a common format: Type (1 octet), Length (2 octets specifying the length of the Value field), Value (variable). The TRILL OAM Application Identifier TLV (Type 64) MUST be the first TLV; an End TLV (Type 0) MUST be the last.
The Authentication TLV (Type 74) with Auth Type 3 contains: Auth Type (1 octet = 3), Key ID (2 octets), and Authentication Data (variable). Coverage extends from the start of the TRILL Header to the end of the OAM Message Channel; TRILL Header Alert bit, Reserved bit, and Hop Count are treated as zero during computation.
The Diagnostic Label TLV (Type 66, Length 5) specifies the data label with: L-Type (1 octet: 0=12-bit 802.1Q VLAN, 1=24-bit fine-grained label), Reserved (1 octet), Label (24 bits). Receiving RBridges MUST compare the Flow Entropy data label to this TLV and set the C (Cross-Connect Error) flag on mismatch.
The Flow Entropy field is a fixed 96-byte field. The rightmost bits MUST be padded with zeros when the actual flow-entropy information is less than 96 bytes. The field begins with Inner.MacDA; its offset depends on whether TRILL extensions are present as specified in RFC 7179.
The Out-of-Band Reply Address TLV (Type 65) carries: Address Type (1 octet: 0=IPv4, 1=IPv6, 2=TRILL nickname), Addr Length (1 octet: 4, 16, or 2 respectively), and Reply Address (variable). When the O bit in the Application Identifier TLV is set, the reply MUST be sent to this address.
The TRILL OAM Application Identifier TLV (Type 64, Length 9) contains: Version (1 octet), Reserved1 (3 octets), Fragment-ID (1 octet, reply only), Return Code (1 octet), Return Sub-code (1 octet), Reserved2 (12 bits), F (Final), C (Cross-Connect Error), O (out-of-band reply requested), and I (in-band reply requested) flags.