TRILL (computing)
Encyclopedia
TRILL is a IETF Standard 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 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
.
) 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:
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 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.
(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. 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.
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.
Link-state routing protocol
A link-state routing protocol is one of the two main classes of routing protocols used in packet switching networks for computer communications . Examples of link-state routing protocols include OSPF and IS-IS....
to the VLAN
Virtual LAN
A virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
-aware customer-bridging problem. RBridges are compatible with and can incrementally replace previous IEEE 802.1
IEEE 802.1
IEEE 802.1 is a working group of the IEEE 802 project of the IEEE Standards Association.It is concerned with:* 802 LAN/MAN architecture* internetworking among 802 LANs, MANs and other wide area networks* 802 Link Security* 802 overall network management...
customer bridges. They are also compatible with IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
and IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
routers and end nodes. They are invisible to current IP routers and, like routers, RBridges terminate the bridge spanning tree protocol
Spanning tree protocol
The Spanning Tree Protocol is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation...
.
General Overview
TRILL devices (RBridgesRouting Bridge
A Routing Bridge or RBridge is a network device that implements the TRILL protocol, as defined by the IETF, and should not be confused with BRouters . RBridges are compatible with previous IEEE 802.1 customer bridges as well as IPv4 and IPv6 routers and end nodes...
) 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
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
because:
- it runs directly over Layer 2Data link layerThe data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
, so it can be run without configuration (no IP addresses need to be assigned) - it is easy to extend by defining new TLVType-length-valueWithin data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol. TLV is also known as tag-length value....
(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
Hop count
In computer networking, hop count refers to the number of routers through which data must pass between source and destination. Each router along the data path constitutes a hop, as the data is moved from one Layer 3 network to another...
. RBridges also specify the next hop RBridge as the frame destination when forwarding unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
frames across a shared-media link, which avoids spawning additional copies of frames during a temporary loop. A Reverse Path Forwarding Check
Reverse path forwarding
Reverse path forwarding is a technique used in modern routers for the purposes of ensuring loop-free forwarding of multicast packets in multicast routing and to help prevent IP address spoofing in unicast routing.- Multicast RPF :...
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
Octet
-Music:* Octet , ensemble consisting of eight instruments or voices, or composition written for such an ensemble* Octet , 1793 composition by Ludwig van Beethoven* Octet , 1825 composition by Felix Mendelssohn...
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 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
Network Layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
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 multi-path routing
Equal-cost multi-path routing is a routing strategy where next-hop packet forwarding to a single destination can occur over multiple "best paths" which tie for top place in routing metric calculations. Multipath routing can be used in conjunction with most routing protocols, since it is a per-hop...
(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 802.3IEEE 802.3
IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...
(Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
) or PPP
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
(Point to Point Protocol). Ethernet links between RBridges can incorporate IEEE customer or provider 802.1 bridges. In other words, an arbitrary bridged LAN
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
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 discoveryLink Layer Discovery Protocol
The Link Layer Discovery Protocol is a vendor-neutral Link Layer protocol in the Internet Protocol Suite used by network devices for advertising their identity, capabilities, and neighbors on a IEEE 802 local area network, principally wired Ethernet...
IEEE 802.1AB, link aggregation
Link aggregation
Link aggregation or trunking or link bundling or Ethernet/network/NIC bonding or NIC teaming are computer networking umbrella terms to describe various methods of combining multiple network connections in parallel to increase throughput beyond what a single connection could sustain, and to provide...
IEEE 802.1AX, MAC security
IEEE 802.1AE
802.1AE is the IEEE MAC Security standard which defines connectionless data confidentiality and integrity for media access independent protocols...
IEEE 802.1AE, or port based access control
IEEE 802.1X
IEEE 802.1X is an IEEE Standard for port-based Network Access Control . It is part of the IEEE 802.1 group of networking protocols. It provides an authentication mechanism to devices wishing to attach to a LAN or WLAN....
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.
External links
- 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 5556, "Transparent Interconnection of Lots of Links (TRILL): Problem and Applicability Statement"
- TRILL Working Group Charter
- Original RBridge paper, “Rbridges: Transparent Routing”