User:Sbarsky/Trill (Computer Networking)

TRILL is an IETF Protocol implemented by devices called RBridges or Routing Bridges. 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 previous IEEE 802.1 customer bridges as well as 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 devices (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: (no IP addresses need to be assigned)
 * it runs directly over Layer 2, so it can be run without configuration
 * 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 2-octet nicknames for RBridges, unique within the campus, which are an abbreviation for the 6-octet IS-IS system ID of the RBridge. The 2-octet nicknames are used to specify the ingress and egress RBridges in the TRILL header.

The TRILL header consists of 6 octets. The first 2 octets include a 6-bit decrementing hop count, plus flags, the next 2 octets contain the egress RBridge nickname, and the final 2 octets contain the ingress RBridge nickname. For multidestination 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 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 built from a variety of link technologies, such as IEEE 802.3 (Ethernet) or PPP (Point to Point Protocol), and can incorporate IEEE 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. 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 (Annex 31B IEEE 802.3), link layer discovery 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 they handle spanning tree and VLAN registration PDUs differently.