Routing Bridges (RBridges): Adjacency
obsoleted by
- rfc-7177 — Transparent Interconnection of Lots of Links (TRILL): Adjacency
updated by
- rfc-7180 — Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates
updates
- rfc-6325 — Routing Bridges (RBridges): Base Protocol Specification
Extracted elements (27)
Adjacency and routing calculations are based solely on connectivity in the link's Designated VLAN because bridged LANs between RBridges may partition VLANs; using the Designated VLAN (dictated by the DRB) ensures a consistent, loop-safe topology view across all RBridges on the link.
Only the DRB 'sprays' TRILL Hellos on all enabled VLANs; non-DRB RBridges send on only a limited set. This avoids the performance cost of up to 4,094× more Hellos while still allowing discovery across VLAN-partitioned bridged links.
The DRB SHOULD bypass the pseudonode on point-to-point-like links because the pseudonode adds complexity: for two RBridges, using a pseudonode generates 3 LSPs whereas bypass generates only 2. Pseudonodes are valuable only when three or more RBridges have simultaneous Report-state adjacencies.
TRILL Hello size is capped at 1,470 octets (not padded to MTU) for loop safety; MTU is tested separately so that a DRB can be uniquely elected even on links with small MTUs. This differs from Layer 3 IS-IS, which pads Hellos to maximum length to block discovery of unreachable neighbors.
TRILL separates the three Hello functions (adjacency formation, DRB election, MTU confirmation) because, unlike Layer 3 IS-IS where multiple Designated Routers on a LAN are tolerable, TRILL requires exactly one DRB per link to prevent native frame loops unprotected by the hop count.
Multiple parallel adjacencies without pseudonodes (point-to-point or LAN adjacencies where no pseudonode is created) in the Report state appear in LSPs as a single adjacency; the cost MAY be adjusted downward to reflect parallel paths.
TRILL uses modified IS-IS LAN subnetwork-dependent functions (Level 1, single Area zero, Ethertype-encoded frames) rather than the standard IS-IS broadcast subnetwork functions (IS-IS Section 8.4), because the TRILL environment requires zero configuration, VLAN awareness, native frame loop safety, and MTU robustness not present in Layer 3 IS-IS.
A DRB SHOULD set the bypass pseudonode bit in its Hellos if it has not seen at least two simultaneous adjacencies in the Report state since it last rebooted or was reset by network management.
All TRILL LAN Hellos issued by an RBridge on a particular port MUST have the same source MAC address, priority, desired Designated VLAN, and Port ID, regardless of the VLAN in which the Hello is sent.
Although MTU testing is optional, it is mandatory for an RBridge to respond to an MTU-probe PDU with an MTU-ack PDU. MTU testing is only performed in the Designated VLAN.
Each TRILL Hello MUST contain an Area Addresses TLV listing only the single Area zero and an MT Port Capabilities TLV containing a VLAN-FLAGS sub-TLV. If a Protocols Supported TLV is present, it MUST list the TRILL NLPID (0xC0).
If multiple RBridge port MAC (SNPA) addresses on a link are the same, all but one of such ports MUST be put in the Suspended state and do not participate in the link except to monitor whether they should remain suspended.
On receipt of a TRILL LAN Hello, if Circuit Type is not 1, Area Address TLV is absent or not Area zero, Protocols Supported TLV lacks TRILL NLPID 0xC0, MT Port Capabilities TLV with VLAN-FLAGS is absent, maximumAreaAddresses is not 1, or IS-IS authentication fails, the PDU is discarded.
RBridges do not implement the IS-IS externalDomain flag, or implement it with the fixed value 'false'; they send and receive TRILL LAN Hellos on every port that is not disabled or configured as point-to-point.
The TRILL Neighbor TLV in a Hello MUST show neighbor information as sensed for the VLAN on which the Hello is sent; implementations maintaining per-VLAN state only for the Designated VLAN send the TLV only in Hellos on that VLAN.
To enable any RBridge to definitively determine 2-way reachability, RB1's neighbor list MUST eventually cover every possible range of SNPA IDs: if X1 is the smallest ID in a fragment without the 'smallest' flag set, X1 MUST also appear as the largest ID in another fragment so no range is ever absent.
TRILL-Hello PDU headers MUST have Maximum Area Addresses equal to 1 and Circuit Type equal to 1.
TRILL-Hello PDUs MUST NOT be sent exceeding 1,470 octets and SHOULD NOT be padded; however, a received TRILL Hello longer than 1,470 octets is processed normally.
DRB election is determined by the 7-bit priority field in Hellos, with MAC (SNPA) address as tiebreaker, Port ID as secondary tiebreaker, and System ID as tertiary tiebreaker; all fields compared as unsigned integers with larger value winning.
Each adjacency table entry records the neighbor SNPA (48-bit MAC), Port ID, and System ID; two 16-bit unsigned Hello holding timers (Designated VLAN and non-Designated VLAN, in seconds); the neighbor's 7-bit DRB priority; and the neighbor's desired Designated VLAN.
Each RBridge LAN port state record includes: an enablement bit, SNPA address, Port ID, Holding Time, DRB priority, DRB status, a 16-bit unsigned Suspension timer (seconds), desired Designated VLAN, and a table of zero or more adjacencies.
MTU testing uses MTU-probe and MTU-ack PDUs. RB1 determines desired campus link MTU as the minimum of all RBridges' originatingL1LSPBufferSize (range 1,470–65,535 bytes in TRILL, not less than 1,470), and reports the largest size passing the MTU test in its TRILL Neighbor TLV.
The pseudonode is controlled by the DRB: if the DRB sets the bypass pseudonode bit in its Hellos, all RBridges report adjacencies directly; otherwise the DRB generates pseudonode LSPs reporting adjacency to all RBridges with Report-state adjacency to the DRB.
TRILL IS-IS frames are distinguished from Layer 3 IS-IS by destination multicast address All-IS-IS-RBridges (01-80-C2-00-00-41) and L2-IS-IS Ethertype (0x22F4), using Ethertype encoding.
This document improves the description of four aspects of the TRILL LAN Hello protocol and does not change the security considerations of the TRILL base protocol. Security considerations from RFC 6325 Section 6 apply, including the use of IS-IS authentication on links: Hellos failing authentication are discarded.
Each RBridge LAN port has a DRB election state: Down (operationally down), Suspended (same MAC as higher-priority port), DRB (port is Designated RBridge), or Not DRB (deferring to another). Transitions are driven by events D1–D5, including suspension timer expiry, adjacency table changes, receipt of higher-priority same-MAC Hello, and port going down.
Each TRILL LAN adjacency passes through four states: Down (non-existent), Detect (neighbor seen but not 2-way on Designated VLAN), 2-Way (bidirectional on Designated VLAN but MTU not confirmed), and Report (full adjacency reported in LSPs). Transitions are driven by events A0–A8, including Hello receipt on/off Designated VLAN, timer expiry, MTU test success/failure, and port going down.