Bridging (networking)
Encyclopedia
Bridging is a forwarding technique used in packet-switched
computer network
s. Unlike routing
, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding
and examination of source addresses in received packet headers to locate unknown devices. Once a device has been located, its location is recorded in a table where the MAC address
is stored so as to avoid the need for further broadcasting. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area network
s.
Bridging generally refers to transparent bridging or learning bridge operation which predominates in Ethernet
. Another form of bridging, source route bridging
, was developed for token ring networks.
A network bridge connects multiple network segment
s at the data link layer
(Layer 2) of the OSI model
. In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D
standard. A bridge and a switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with bridge.
As an example, consider three hosts, A, B and C and a bridge. The bridge has three ports. A is connected to bridge port 1, B is connected 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 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 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. 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.
Note that both source and destination addresses are used in this algorithm. 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.
The technology was originally developed by the Digital Equipment Corporation
(DEC) in the 1980s.
and decides to either forward or filter. If the bridge determines that the destination node
is on another segment on the network, it forwards (retransmits) the packet to that segment. If the destination address belongs to the same segment as the source address, the bridge filters (discards) the frame. As nodes transmit data through the bridge, the bridge establishes a filtering database (also known as a forwarding table) of known MAC addresses and their locations on the network. The bridge uses its filtering database to determine whether a packet should be forwarded or filtered.
in order to switch it to the appropriate port/s. In case that MAC address
exists in the Switch L2 Table, it transmits the Frame only to the port which is tied to that entry. In case that MAC address doesn’t exist in the Switch L2 Table, the frame is considered DLF and it been transmitted to all forwarding ports of that VLAN. (Also Broadcasts such as ARP Request messages are transmitted to the same ports)
are both ways of performing data control, but work through different methods. Bridging takes place at OSI Model Layer 2 (data-link layer) while routing takes place at the OSI Model Layer 3 (network layer). This difference means that a bridge directs frames according to hardware assigned MAC address
es while a router makes its decisions according to arbitrarily assigned IP Address
es. As a result of this, bridges are not concerned with and are unable to distinguish networks while routers can.
When designing a network, one can choose to put multiple segments into one bridged network or to divide it into different networks interconnected by routers. If a host is physically moved from one network area to another in a routed network, it has to get a new IP address; if this system is moved within a bridged network, it doesn't have to reconfigure anything.
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s. Unlike routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding
Flooding (computer networking)
Flooding is a simple routing algorithm in which every incoming packet is sent through every outgoing link. See flooding algorithm for details.-Advantages:* If a packet can be delivered, it will ....
and examination of source addresses in received packet headers to locate unknown devices. Once a device has been located, its location is recorded in a table where the MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
is stored so as to avoid the need for further broadcasting. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area network
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...
s.
Bridging generally refers to transparent bridging or learning bridge operation which predominates in 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....
. Another form of bridging, source route bridging
Source route bridging
Source route bridging is used on token ring networks, and is standardized in Section 9 of the IEEE 802.2 standard. The operation of the bridge is simpler and much of the bridging functions are performed by the end systems, particularly the sources, giving rise to its name.Source-route transparent...
, was developed for token ring networks.
A network bridge connects multiple network segment
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:...
s at the data link layer
Data link layer
The 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....
(Layer 2) of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D
IEEE 802.1D
802.1D is the IEEE MAC Bridges standard which includes Bridging, Spanning Tree and others. It is standardized by the IEEE 802.1 working group. It includes details specific to linking many of the other 802 projects including the widely deployed 802.3 , 802.11 and 802.16 standards.VLANs are not...
standard. A bridge and a switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with bridge.
Transparent bridging operation
A bridge uses a forwarding database to send frames across network segments. The forwarding database is initially empty and entries in the database are built as the bridge receives frames. If an address entry is not found in the forwarding database, the frame is flooded to all other ports of the bridge, forwarding the frame to all segments except the source address. By means of these broadcast frames, the destination network will respond and a forwarding database entry will be created.As an example, consider three hosts, A, B and C and a bridge. The bridge has three ports. A is connected to bridge port 1, B is connected 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 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 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. 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.
Note that both source and destination addresses are used in this algorithm. 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.
The technology was originally developed by the Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
(DEC) in the 1980s.
Filtering database
To translate between two segments, a bridge reads a frame's destination MAC addressMAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
and decides to either forward or filter. If the bridge determines that the destination node
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
is on another segment on the network, it forwards (retransmits) the packet to that segment. If the destination address belongs to the same segment as the source address, the bridge filters (discards) the frame. As nodes transmit data through the bridge, the bridge establishes a filtering database (also known as a forwarding table) of known MAC addresses and their locations on the network. The bridge uses its filtering database to determine whether a packet should be forwarded or filtered.
Destination lookup failure
Layer 2 (L2) Ethernet Switch is looking at the MAC Destination address of the Ethernet frameEthernet frame
A data packet on an Ethernet link is called an Ethernet frame. A frame begins with Preamble and Start Frame Delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any...
in order to switch it to the appropriate port/s. In case that MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
exists in the Switch L2 Table, it transmits the Frame only to the port which is tied to that entry. In case that MAC address doesn’t exist in the Switch L2 Table, the frame is considered DLF and it been transmitted to all forwarding ports of that VLAN. (Also Broadcasts such as ARP Request messages are transmitted to the same ports)
Advantages of network bridges
- Simple bridges are inexpensive
- Isolate collision domains with microsegmentationMicrosegmentationMicrosegmentation in computer networking is a term used to describe the segmentation of a collision domain into as many segments as there are circuits, minus one . This microsegmentation performed by the switch cuts the collision domain down so that only two nodes coexist within each collision domain...
- Access controlNetwork Access ControlNetwork Access Control is an approach to computer network security that attempts to unify endpoint security technology , user or system authentication and network security enforcement.-Background:Network Access Control is a computer networking solution that uses a set of protocols to define and...
and network managementNetwork managementNetwork management refers to the activities, methods, procedures, and tools that pertain to the operation, administration, maintenance, and provisioning of networked systems....
capabilities - Bandwidth scales as network grows
Disadvantages of network bridges
- Does not limit the scope of broadcasts
- Does not scale to extremely large networks
- Buffering and processing introduces delays
- A complex network topology can pose a problem for transparent bridges. For example, multiple paths between transparent bridges and LANsLocal area networkA local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
can result in bridge loops. The spanning tree protocolSpanning tree protocolThe 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...
helps to reduce problems with complex topologies.
Bridging versus routing
Bridging and routingRouting
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
are both ways of performing data control, but work through different methods. Bridging takes place at OSI Model Layer 2 (data-link layer) while routing takes place at the OSI Model Layer 3 (network layer). This difference means that a bridge directs frames according to hardware assigned MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
es while a router makes its decisions according to arbitrarily assigned IP Address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
es. As a result of this, bridges are not concerned with and are unable to distinguish networks while routers can.
When designing a network, one can choose to put multiple segments into one bridged network or to divide it into different networks interconnected by routers. If a host is physically moved from one network area to another in a routed network, it has to get a new IP address; if this system is moved within a bridged network, it doesn't have to reconfigure anything.
See also
- Collision domainCollision domainA collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular, when using early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network...
- Instrument bridgeInstrument bridgeAn Instrument bridge is an extension of the network bridge concept from the IEEE networking technology glossary. The term instrument bridge highlights the logical separation of scientific instruments from administrative computers An Instrument bridge is an extension of the network bridge concept...
- Promiscuous modePromiscuous modeIn computer networking, promiscuous mode or promisc mode is a mode for a network interface controller that causes the NIC to pass all traffic it receives to the central processing unit rather than just passing frames the NIC is intended to receive...
- Spanning Tree ProtocolSpanning tree protocolThe 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...
- Wireless bridgeWireless bridgeA wireless bridge is a hardware component used to connect two or more network segments which are physically and logically separated. It does not necessarily always need to be a hardware device, as some operating systems provide software to bridge different protocols...
- Audio Video BridgingAudio Video BridgingAudio Video Bridging is a common name for the set of technical standards developed by the Institute of Electrical and Electronics Engineers Audio Video Bridging Task Group of the IEEE 802.1 standards committee. The charter of this organization is to "provide the specifications that will allow...