![Network Bridging](https://upload.wikimedia.org/wikipedia/commons/e/e8/Network_Bridging.png)
A network bridge is a
computer networking device
Networking hardware, also known as network equipment or computer networking devices, are electronic devices which are required for communication and interaction between devices on a computer network. Specifically, they mediate data transmission in ...
that creates a single, aggregate network from multiple
communication network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, messag ...
s or
network segment
A network segment is a portion of a computer network. The nature and extent of a segment depends on the nature of the network and the device or devices used to interconnect end stations.
Ethernet
According to the defining IEEE 802.3 standards ...
s. This function is called network bridging. Bridging is distinct from
routing
Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...
. Routing allows multiple networks to communicate independently and yet remain separate, whereas bridging connects two separate networks as if they were a single network. In the
OSI model
The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
, bridging is performed in the
data link layer
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
(layer 2). If one or more segments of the bridged network are
wireless
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
, the device is known as a wireless bridge.
The main types of network bridging technologies are simple bridging, multiport bridging, and learning or transparent bridging.
Transparent bridging
Transparent bridging uses a table called the
forwarding information base
A forwarding information base (FIB), also known as a forwarding table or MAC table, is most commonly used in network bridging, routing, and similar functions to find the proper output network interface controller to which the input interface shou ...
to control the forwarding of frames between network segments. The table starts empty and entries are added as the bridge receives frames. If a destination address entry is not found in the table, the frame is flooded to all other ports of the bridge, flooding the frame to all segments except the one from which it was received. By means of these flooded frames, a
host
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
* Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
* Michel Host ...
on the destination network will respond and a forwarding database entry will be created. Both source and destination addresses are used in this process: source addresses are recorded in entries in the table, while destination addresses are looked up in the table and matched to the proper segment to send the frame to.
Digital Equipment Corporation
Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...
(DEC) originally developed the technology in the 1980s.
In the context of a two-port bridge, the forwarding information base can be seen as a filtering database. A bridge reads a
frame
A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent.
Frame and FRAME may also refer to:
Physical objects
In building construction
*Framing (con ...
's destination address and decides to either forward or filter. If the bridge determines that the destination host is on another segment on the network, it forwards the frame to that segment. If the destination address belongs to the same segment as the source address, the bridge filters the frame, preventing it from reaching the other network where it is not needed.
Transparent bridging can also operate over devices with more than two ports. As an example, consider a bridge connected to three hosts, A, B, and C. The bridge has three ports. A is connected to bridge port 1, B is connected to bridge port 2, C is connected to bridge port 3. A sends a frame addressed to B to the bridge. The bridge examines the source address of the frame and creates an address and port number entry for host ''A'' in its forwarding table. The bridge examines the destination address of the frame and does not find it in its forwarding table so it floods (broadcasts) it to all other ports: 2 and 3. The frame is received by hosts B and C. Host C examines the destination address and ignores the frame as it does not match with its address. Host B recognizes a destination address match and generates a response to A. On the return path, the bridge adds an address and port number entry for B to its forwarding table. The bridge already has A's address in its forwarding table so it forwards the response only to port 1. Host C or any other hosts on port 3 are not burdened with the response. Two-way communication is now possible between A and B without any further flooding to the network. Now, if A sends a frame addressed to C, the same procedure will be used, but this time the bridge will not create a new forwarding-table entry for A's address/port because it has already done so.
Bridging is called ''transparent'' when the frame format and its addressing aren't changed substantially. ''Non-transparent'' bridging is required especially when the frame addressing schemes on both sides of a bridge are not compatible with each other, e.g. between
ARCNET
Attached Resource Computer NETwork (ARCNET or ARCnet) is a communications protocol for local area networks. ARCNET was the first widely available networking system for microcomputers; it became popular in the 1980s for office automation tasks. It ...
with local addressing and
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
using IEEE
MAC addresses
A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking techno ...
, requiring translation. However, most often such incompatible networks are
routed in between, not bridged.
Simple bridging
A simple bridge connects two network segments, typically by operating transparently and deciding on a frame-by-frame basis whether or not to forward from one network to the other. A
store and forward
Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in ...
technique is typically used so, as part of forwarding, the frame integrity is verified on the source network and
CSMA/CD
Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other statio ...
delays are accommodated on the destination network. In contrast to repeaters which simply extend the maximum span of a segment, bridges only forward frames that are required to cross the bridge. Additionally, bridges reduce collisions by creating a separate
collision domain
In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...
on either side of the bridge.
Multiport bridging
A multiport bridge connects multiple networks and operates transparently to decide on a frame-by-frame basis ''whether'' to forward traffic. Additionally, a multiport bridge must decide ''where'' to forward traffic. Like the simple bridge, a multiport bridge typically uses store and forward operation. The multiport bridge function serves as the basis for
network switch
A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device.
A netw ...
es.
Implementation
The
forwarding information base
A forwarding information base (FIB), also known as a forwarding table or MAC table, is most commonly used in network bridging, routing, and similar functions to find the proper output network interface controller to which the input interface shou ...
stored in
content-addressable memory
Content-addressable memory (CAM) is a special type of computer memory used in certain very-high-speed searching applications. It is also known as associative memory or associative storage and compares input search data against a table of stored d ...
(CAM) is initially empty. For each received
Ethernet frame
In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload ...
the switch learns from the frame's source MAC address and adds this together with an interface identifier to the forwarding information base. The switch then forwards the frame to the interface found in the CAM based on the frame's destination MAC address. If the destination address is unknown the switch sends the frame out on all interfaces (except the ingress interface). This behavior is called
unicast flood
In computer networking, a unicast flood is when a switch receives a unicast frame and treats it as a broadcast frame, flooding the frame to all other ports on the switch.
Background
The term ''unicast'' refers to a one-to-one transmission from one ...
ing.
Forwarding
Once a bridge learns the addresses of its connected nodes, it forwards data link layer frames using a layer-2 forwarding method. There are four forwarding methods a bridge can use, of which the second through fourth methods were performance-increasing methods when used on "switch" products with the same input and output port bandwidths:
#
Store and forward
Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in ...
: the switch buffers and verifies each frame before forwarding it; a frame is received in its entirety before it is forwarded.
#
Cut through: the switch starts forwarding after the frame's destination address is received. There is no error checking with this method. When the outgoing port is busy at the time, the switch falls back to store-and-forward operation. Also, when the egress port is running at a faster data rate than the ingress port, store-and-forward is usually used.
#
Fragment free
Fragment may refer to:
Entertainment
Television and film
* "Fragments" (''Torchwood''), an episode from the BBC TV series
* "Fragments", an episode from the Canadian TV series ''Sanctuary''
* "Fragments" (Steven Universe Future), an episode f ...
: a method that attempts to retain the benefits of both store and forward and cut through. Fragment free checks the first 64
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
s of the frame, where
addressing information is stored. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frame transmissions that are aborted because of a collision will not be forwarded. Error checking of the actual data in the packet is left for the end device.
#
Adaptive switching
An adaptive switch is a network switch designed to normally operate in cut-through mode but if a port's error rate jumps too high, the switch automatically reconfigures the port to run in store-and-forward mode. This optimizes the switch's perfor ...
: a method of automatically selecting between the other three modes.
Shortest Path Bridging
Shortest Path Bridging (SPB), specified in the IEEE 802.1aq standard and based on
Dijkstra's algorithm
Dijkstra's algorithm ( ) is an algorithm for finding the shortest paths between nodes in a graph, which may represent, for example, road networks. It was conceived by computer scientist Edsger W. Dijkstra in 1956 and published three years ...
, is a
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...
ing technology intended to simplify the creation and configuration of networks, while enabling
multipath routing
Multipath routing is a routing technique simultaneously using multiple alternative paths through a network. This can yield a variety of benefits such as fault tolerance, increased bandwidth, and improved security.
Mobile networks
To improve pe ...
. It is a proposed replacement for
Spanning Tree Protocol
The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also al ...
which blocks any redundant paths that could result in a
switching loop
A switching loop or bridge loop occurs in computer networks when there is more than one layer 2 path between two endpoints (e.g. multiple connections between two network switches or two ports on the same switch connected to each other). The loop ...
. SPB allows all paths to be active with multiple equal-cost paths. SPB also increases the number of VLANs allowed on a layer-2 network.
TRILL
TRILL (Transparent Interconnection of Lots of Links) is an Internet Standard implemented by devices called TRILL switches. TRILL combines techniques from bridging and routing, and is the application of link-state routing to the VLAN-aware cus ...
(TRansparent Interconnection of Lots of Links) is the successor to Spanning Tree Protocol, both having been created by the same person,
Radia Perlman
Radia Joy Perlman (; born December 18, 1951) is an American computer programmer and network engineer. She is a major figure in assembling the networks and technology to enable what we now know as the internet. She is most famous for her inventi ...
. The catalyst for TRILL was an event at
Beth Israel Deaconess Medical Center
Beth Israel Deaconess Medical Center (BIDMC) in Boston, Massachusetts is a teaching hospital of Harvard Medical School. It was formed out of the 1996 merger of Beth Israel Hospital (founded in 1916) and New England Deaconess Hospital (founded ...
which began on 13 November 2002. The concept of Rbridges
icwas first proposed to the
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operation ...
in the year 2004, whom in 2005 rejected what came to be known as TRILL, and in the years 2006 through 2012
devised an incompatible variation known as Shortest Path Bridging.
See also
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References
{{DEFAULTSORT:Bridging (Networking)
Network architecture
Ethernet
Bridge
A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...