|Internet protocol suite|
TRILL (Transparent Interconnection of Lots of Links) is an IETF Standard implemented by devices called RBridges (Routing Bridges) or TRILL Switches. TRILL combines the advantages of bridges and routers and is the application of link state routing to the VLAN-aware customer-bridging problem. RBridges are compatible with and can incrementally replace previous IEEE 802.1 customer bridges. They are also compatible with IPv4 and IPv6 routers and end nodes. They are invisible to current IP routers and, like routers, RBridges terminate the bridge spanning tree protocol.
General Overview 
TRILL switches (RBridges) run a link state protocol amongst themselves. A link state protocol is one in which connectivity is broadcast to all the RBridges, so that each RBridge knows about all the other RBridges, and the connectivity between them. This gives RBridges enough information to compute pair-wise optimal paths for unicast, and calculate distribution trees for delivery of frames either to destinations whose location is unknown or to multicast / broadcast groups. The link state routing protocol used is IS-IS because:
- it runs directly over Layer 2, so it can be run without configuration (no IP addresses need to be assigned)
- it is easy to extend by defining new TLV (type-length-value) data elements and sub-elements for carrying TRILL information.
To mitigate temporary loop issues, RBridges forward based on a header with a hop count. RBridges also specify the next hop RBridge as the frame destination when forwarding unicast frames across a shared-media link, which avoids spawning additional copies of frames during a temporary loop. A reverse path forwarding check and other checks are performed on multi-destination frames to further control potentially looping traffic.
The first RBridge that a unicast frame encounters in a campus, RB1, encapsulates the received frame with a TRILL header that specifies the last RBridge, RB2, where the frame is decapsulated. RB1 is known as the "ingress RBridge" and RB2 is known as the "egress RBridge". To save room in the TRILL header and simplify forwarding lookups, a dynamic nickname acquisition protocol is run among the RBridges to select two-octet nicknames for RBridges, unique within the campus, which are an abbreviation for the six-octet IS-IS system ID of the RBridge. The two-octet nicknames are used to specify the ingress and egress RBridges in the TRILL header.
The TRILL header consists of six octets: the first two octets include a six bit decrementing hop count, plus flags; the next two octets contain the egress RBridge nickname; the final two octets contain the ingress RBridge nickname. For multi-destination frames, the "egress RBridge nickname" specifies a distribution tree for the frame, where the (nick)named RBridge is the root of the distribution tree. The ingress RBridge selects which distribution tree the frame should travel along.
Even though RBridges are transparent to Layer 3 devices, and all the links interconnected by RBridges appear to Layer 3 devices to be a single link, RBridges act as link routers in the sense that, in the forwarding of a frame by a transit RBridge, the outer Layer 2 header is replaced at each hop with an appropriate Layer 2 header for the next hop, and the hop count is decreased. Despite these modifications of the outer Layer 2 header and the hop count in the TRILL Header, the original encapsulated frame is preserved, including the original frame's VLAN tag.
Multipathing of multi-destination frames through alternative distribution tree roots and ECMP (Equal Cost MultiPath) of unicast frames are supported. Networks with a more mesh-like structure will benefit to a greater extent from the multipathing and optimal paths provided by TRILL than will networks with a more tree-like structure.
TRILL Links 
From the point of view of TRILL, a link can be any of a wide variety of link technologies, including as IEEE 802.3 (Ethernet) or PPP (Point to Point Protocol). Ethernet links between RBridges can incorporate IEEE customer or provider 802.1 bridges. In other words, an arbitrary bridged LAN appears to an RBridge as a multi-access link.
It is essential that only one RBridge act as the ingress RBridge for any given frame and TRILL has an Appointed Forwarder  mechanism to assure this. TRILL does allow load splitting of this duty on a link based on VLAN, so that only one RBridge on each link encapsulates and decapsulates native frames for each VLAN.
RBridge Ports 
RBridge ports can compatibly implement a wide variety of existing and proposed link level and IEEE 802.1 port level protocols including PAUSE (IEEE 802.3 Annex 31B), the Link Layer Discovery Protocol (IEEE 802.1AB), link aggregation (IEEE 802.1ax), MAC security (IEEE 802.1AE), or port based access control (IEEE 802.1X). This is because RBridges are layered above the IEEE 802.1 EISS (Extended Internal Sublayer Service) with the exception that an RBridge port handles spanning tree and VLAN registration PDUs differently.
TRILL limited to 4096 VLANs but standardization work is being done on Fine Grain Labeling (FGL) that will remove this restriction.
See also 
- RFC 5556, "Transparent Interconnection of Lots of Links (TRILL): Problem and Applicability Statement"
- RFC 6325 "Routing Bridges (RBridges): Base Protocol Specification"
- RFC 6326 "Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS"
- RFC 6327 "Routing Bridges (RBridges): Adjacency"
- RFC 6361 "PPP Transparent Interconnection of Lots of Links (TRILL) Protocol Control Protocol " (TRILL over PPP)
- RFC 6439 "Routing Bridges (RBridges): Appointed Forwarders"
- RFC 6847 "Fibre Channel over Ethernet (FCoE) over Transparent Interconnection of Lots of Links (TRILL)"
- RFC 6850 "Definitions of Managed Objects for Routing Bridges (RBridges)"
- TRILL Working Group Charter
- "Introduction to Trill" by Radia Perlman and Donald Eastlake
- Original RBridge paper, “Rbridges: Transparent Routing”