Network bridge: Difference between revisions

Revision as of 21:50, 18 September 2005

Latest revision as of 15:23, 1 August 2025

(557 intermediate revisions by more than 100 users not shown)

Line 1:

{{Short description|Device that creates a larger computer network from two smaller networks}}

''Bridging'' is a [[forwarding]] technique use in [[packet switching|packet-switched]] [[computer network|computer networks]].

{{Use American English|date = March 2019}}

Unlike [[routing]], bridging makes no assumptions about where in a network a particular address

{{Use mdy dates|date = March 2019}}

is located. Instead, it depends on broadcasting to locate unknown devices. Once

a device has been located, its ___location is recorded in some fashion so as to

preclude the need for further broadcasting.

[[File:Network Bridging.png|thumb|right|upright=1.8|A high-level overview of network bridging, using the [[OSI model|ISO/OSI layers]] and terminology]]

The utility of bridging is limited by its dependence on broadcasting, and is thus

used only in small networks, or small subsets of large networks.

A '''network bridge''' is a [[computer networking device]] that creates a single, aggregate network from multiple [[communication network]]s or [[network segment]]s. This function is called '''network bridging'''.<ref>{{cite web |url=https://learningnetwork.cisco.com/servlet/JiveServlet/previewBody/2810-102-1-7611/primch5.pdf |title=Traffic regulators: Network interfaces, hubs, switches, bridges, routers, and firewalls |publisher=[[Cisco Systems]] |date=1999-09-14 |access-date=2012-07-27 |archive-url=https://web.archive.org/web/20130531002143/https://learningnetwork.cisco.com/servlet/JiveServlet/previewBody/2810-102-1-7611/primch5.pdf |archive-date=May 31, 2013 |url-status=dead }}</ref> Bridging is distinct from [[routing]]. Routing allows multiple networks to communicate independently and yet remain separate, whereas bridging connects two separate networks as if they were a single network.<ref>{{cite web |url=http://www.cisco.com/cisco/web/solutions/small_business/resource_center/articles/connect_employees_and_offices/what_is_a_network_switch/index.html |title=What is a Network Switch vs. a Router? |publisher=[[Cisco Systems]] |access-date=2012-07-27}}</ref> In the [[OSI model]], bridging is performed in the [[data link layer]] (layer 2).<ref>{{cite web|url=http://tools.ietf.org/html/rfc1286|title=RFC 1286 - Definitions of Managed Objects for Bridges|date=1989-07-14|publisher=Tools.ietf.org|access-date=2013-10-19 |last1=Decker |first1=Eric B. |last2=Langille |first2=Paul |last3=McCloghrie |first3=Keith |last4=Rijsinghani |first4=Anil }}</ref> If one or more segments of the bridged network are [[Wireless network|wireless]], the device is known as a '''wireless bridge'''.

Currently, two different bridging technologies are in widespread use.

''Transparent bridging'' predominates in [[Ethernet]] networks;

while ''source routing'' is used in [[Token ring|Token-Ring]] networks.

The main types of network bridging technologies are simple bridging, multiport bridging, and learning or transparent bridging.<ref>{{cite web|url=http://manipalitdubai.com/material/Lecture_Notes/ISM201/Internetworking_concept.ppt|archive-url=https://web.archive.org/web/20140513184912/http://manipalitdubai.com/material/Lecture_Notes/ISM201/Internetworking_concept.ppt |format=PowerPoint |title=Local Area Networks: Internetworking |publisher=manipalitdubai.com |access-date=2012-12-02 |archive-date=2014-05-13}}</ref><ref>{{cite web |url=https://www.iol.unh.edu/sites/default/files/knowledgebase/bfc/UNH-IOL_BFC_Knowledgebase_Bridging.ppt |format=PowerPoint |title=Bridging Protocols Overview |publisher=iol.unh.edu |access-date=2012-12-02}}</ref>

==Transparent Bridging==

''== {{Anchor|TRANSPARENT}}Transparent bridging'' refers to a form of bridging "transparent" to the==

Transparent bridging uses a table called the ''[[forwarding information base]]'' 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 forwarded 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 (network)|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.<ref>{{cite web

end systems using it, in the sense that the end systems operate in

|url= http://docwiki.cisco.com/wiki/Transparent_Bridging

exactly the same way as in a non-bridged network.

|title= Transparent Bridging

It is used primarily in Ethernet networks, where it has

|publisher= Cisco Systems, Inc.

been standardized as [[IEEE 802.1d]].

|access-date= 2010-06-20

|archive-date= November 21, 2015

|archive-url= https://web.archive.org/web/20151121211144/http://docwiki.cisco.com/wiki/Transparent_Bridging

|url-status= dead

}}</ref> [[Digital Equipment Corporation]] (DEC) originally developed the technology in 1983<ref>{{cite patent |title=Bridge circuit for interconnecting networks |country=US |number=4597078}}</ref> and introduced the LANBridge 100 that implemented it in 1986.

<ref>{{cite web

|url= https://spectrum.ieee.org/how-dec-engineers-saved-ethernet

|title= How Engineers at Digital Equipment Corp. Saved Ethernet

|publisher= IEEE Spectrum

|access-date= 2024-04-10

|date= 2024-04-07}}</ref>

In the context of a two-port bridge, the forwarding information base can be seen as a filtering database. A bridge reads a [[Frame (telecommunications)|frame]]'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.

The bridging

functions are confined to [[network bridge|network bridges]] which interconnect

the [[network segments]]. First, the network bridges use

the [[spanning tree (networks)|spanning tree]] algorithm to built a loop-free network

topology by selectively disabling network segments.

This allows broadcasting

to occur simply by copying packets;

the tree structure insures that loops will

not occur and that broadcast packets will

therefore not be copied indefinately.

Next, the bridges monitor all packets traveling

on the network, noting their source addresses,

matching these addresses to the interfaces

the packets were received on, and recording this

information in a table. Finally, the bridges

forward packets according to a simple algorithm

- if the destination address does not appear

in the table, broadcast the packet throughout

the network; otherwise, forward it only on the

interface recording in the table.

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.

Note that both source and destination addresses

are used in this algorithm. Source addresses

are used to record entries in the table, while

destination addresses are used to lookup entries.

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]] with local addressing and [[Ethernet]] using IEEE [[MAC addresses]], requiring translation. However, most often such incompatible networks are [[routed]] in between, not bridged.

As a simple example, consider a packet exchange between

two hosts (A and B) in a bridged network.

A begins by transmitting a packet to B.

Since B (the destination) does not appear in

any of the bridges's tables, the packet

is broadcast throughout the network.

Each network bridge sees a copy of the packet,

and records its source address (A) along

with the interface it was received on.

Each bridge now has an entry for A in its

table. B, having received a packet from A,

now transmits a packet in response. Since

the bridges all have an entry for A (the

packet's destination), no broadcast is

required, and the packet is forwarded

along the tree from B to A. All the bridges

along this path record the packet's source

address - B. Two-way communication is

now possible between A and B without any

further broadcasting. Note, however, that

only the bridges along the direct path

between A and B possess table entries for B.

If a third host (C), not along the direct

path, now transmits a packet for B, it

will be broadcast throughout

the network.

== {{Anchor|SIMPLE}}Simple bridging ==

==Source Route 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]] technique is typically used so, as part of forwarding, the frame integrity is verified on the source network and [[CSMA/CD]] 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]] on either side of the bridge.

== {{Anchor|MULTIPORT}}Multiport bridging ==

''Source route bridging'' is used primarily on

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]]es.

[[Token-Ring]] networks, and

is standardized in Section 9 of the [[IEEE 802.2]]

standard. The [[spanning-tree]]

algorithm is not used, the operation of the

[[network bridge|network bridges]] is simplier, and much of

the bridging functions are performed by the

end systems, particularly the sources, giving rise to its name.

==Implementation==

A field in the Token-Ring header, the

The [[forwarding information base]] stored in [[content-addressable memory]] (CAM) is initially empty. For each received [[Ethernet frame]] 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]]ing.

Routing Information Field (RIF) is used to

support source-route bridging. Upon sending

a packet, a host attaches a RIF to the packet

indicating the series of bridges and network segments

to be used for deliverying the packet to its

destination. The bridges mearly follow the list

given in the RIF - if a given bridge is next

in the list, it forwards the packet, otherwise it

ignores it.

==Forwarding==

When a host wishes to send a packet to a destination

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:

for the first time, it needs to determine an appropriate

RIF. A special type of broadcast packet is used,

which instructs the network bridges to append their

bridge number and network segment number to each

packet as it is forwarded. Loops are avoided by

requiring each bridge to ignore packets which

already contain its bridge number in the RIF field.

At the destination, these broadcast packets are

modified to be standard unicast packets and returned

to the source along the reverse path listed in

the RIF. Thus, for each route discovery packet

broadcast, the source receives back a set of

packets, once for each possible path through

the network to the destination. It is then

up to the source to choice one of these paths

(probably the shortest one) for further

communications with the destination.

# [[Store and forward]]: the switch buffers and verifies each frame before forwarding it; a frame is received in its entirety before it is forwarded.

== Source Routing Transparent (SRT) Bridging==

# [[Cut-through switching|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]]: a method that attempts to retain the benefits of both store and forward and cut through. Fragment free checks the first 64 [[byte]]s of the frame, where [[MAC address|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]]: a method of automatically selecting between the other three modes.<ref name="Dong">{{cite book|last1=Dong|first1=Jielin|title=Network Dictionary|publisher=Javvin Technologies Inc.|isbn=9781602670006|page=23|url=https://books.google.com/books?id=On_Hh23IXDUC&q=adaptive+switching+network&pg=PA23|access-date=25 June 2016|language=en|year=2007}}</ref><ref>{{cite web|title=Cray makes its Ethernet switches responsive to net conditions|url=https://books.google.com/books?id=6xcEAAAAMBAJ&q=adaptive+switching+network&pg=PA8|publisher=IDG Network World Inc|access-date=25 June 2016|language=en|date=1 July 1996}}</ref>

==Shortest Path Bridging==

''Source routing transparent'' (SRT) bridging

[[Shortest Path Bridging]] (SPB), specified in the IEEE 802.1aq standard and based on [[Dijkstra's algorithm]], is a [[computer network]]ing technology intended to simplify the creation and configuration of networks, while enabling [[multipath routing]].<ref>

is a hybrid of source routing and transparent

{{cite news

bridging, standardized in Section 9

|title = Alcatel-Lucent, Avaya, Huawei, Solana and Spirent Showcase Shortest Path Bridging Interoperability

of the [[IEEE 802.2]] standard.

|publisher = Huawei

It allows source routing and transparent

|date = 7 September 2011

bridging to coexist on the same

|url = http://www.marketwire.com/press-release/alcatel-lucent-avaya-huawei-solana-spirent-showcase-shortest-path-bridging-interoperability-paris-alu-1557944.htm

bridged network by using source routing

|access-date = 11 September 2011

with hosts than support it and transparent

}}

bridging otherwise.

</ref><ref>

{{cite book

|chapter = An improved shortest path bridging protocol for Ethernet backbone network

|publisher = IEEE Xplore

|issn = 1976-7684

|isbn = 978-1-61284-661-3

|doi = 10.1109/ICOIN.2011.5723169

|date = 3 March 2011

|title = The International Conference on Information Networking 2011 (ICOIN2011)

|pages = 148–153

|last1 = Luo

|first1 = Zhen

|last2 = Suh

|first2 = Changjin

|s2cid = 11193141

}}

</ref><ref>

{{cite web

|title = Lab Testing Summary Report; Data Center Configuration with SPB

|publisher = Miercom

|date=September 2011

|url = http://docs.media.bitpipe.com/io_10x/io_101870/item_458574/Miercom%20Report%20Avaya%20Ethernet%20Fabric%20SR111013%2015Oct11%20%282%29.pdf

|access-date = 25 December 2011

}}

</ref> It is a proposed replacement for [[Spanning Tree Protocol]] which blocks any redundant paths that could result in a [[switching 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.<ref>

{{cite web

|title = IEEE approves new IEEE 802.1aq™ Shortest path bridging

|publisher = IEEE Standards Association

|author = Shuang Yu

|quote = Using the IEEE’s next-generation VLAN, called a Service Interface Identifier (I-SID), it is capable of supporting 16 million unique services compared to the VLAN limit of four thousand.

|url = http://standards.ieee.org/news/2012/802.1aq.html

|archive-url = https://web.archive.org/web/20130514211405/http://standards.ieee.org/news/2012/802.1aq.html

|url-status = dead

|archive-date = May 14, 2013

|access-date = 19 June 2012

}}

</ref>

[[TRILL]] (Transparent Interconnection of Lots of Links) is the successor to Spanning Tree Protocol, both having been created by the same person, [[Radia Perlman]]. The catalyst for TRILL was an event at [[Beth Israel Deaconess Medical Center]] which began on 13 November 2002.<ref>{{cite web |title=All Systems Down |url=https://community.cisco.com/legacyfs/online/legacy/0/9/8/140890-All%20Systems%20Down%20-%20Scott%20Berinato(CIO).pdf |website=cio.com |publisher=IDG Communications, Inc. |access-date=9 January 2022 |archive-url=https://web.archive.org/web/20200923200221if_/https://community.cisco.com/legacyfs/online/legacy/0/9/8/140890-All%20Systems%20Down%20-%20Scott%20Berinato(CIO).pdf |archive-date=23 September 2020 |url-status=dead}}</ref><ref>{{cite web |title=All Systems Down |url=https://www.computerworld.com/article/2581420/all-systems-down.html |website=cio.com |publisher=IDG Communications, Inc. |access-date=9 January 2022 |archive-url=https://web.archive.org/web/20220109020703/https://www.computerworld.com/article/2581420/all-systems-down.html |archive-date=9 January 2022 |url-status=dead}}</ref> The concept of Rbridges<ref>{{cite web |title=Rbridges: Transparent Routing |url=https://courses.cs.washington.edu/courses/cse590l/05sp/papers/rbridges.pdf |website=courses.cs.washington.edu |publisher=Radia Perlman, Sun Microsystems Laboratories |access-date=9 January 2022 |archive-url=https://web.archive.org/web/20220109030037/https://courses.cs.washington.edu/courses/cse590l/05sp/papers/rbridges.pdf |archive-date=9 January 2022 |url-status=dead}}</ref> [sic] was first proposed to the [[Institute of Electrical and Electronics Engineers]] in the year 2004,<ref>{{cite web |title=Rbridges: Transparent Routing |url=https://www.researchgate.net/publication/4102976 |website=researchgate.net |publisher=Radia Perlman, Sun Microsystems; Donald Eastlake 3rd, Motorola}}</ref> who in 2005<ref>{{cite web |title=TRILL Tutorial |url=http://www.postel.org/rbridge/trill-tutorial.pdf |website=postel.org |publisher=Donald E. Eastlake 3rd, Huawei }}</ref> rejected what came to be known as TRILL, and in the years 2006 through 2012<ref>{{cite web |title=IEEE 802.1: 802.1aq - Shortest Path Bridging |url=https://ieee802.org/1/pages/802.1aq.html |website=ieee802.org |publisher=Institute of Electrical and Electronics Engineers }}</ref> devised an incompatible variation known as Shortest Path Bridging.

== See also ==

* {{annotated link|Audio Video Bridging}}

* {{annotated link|IEEE 802.1D}}

* {{annotated link|IEEE 802.1Q}}

* {{annotated link|IEEE 802.1ah-2008}}

* {{annotated link|Promiscuous mode}}

== References ==

{{Reflist}}

{{Authority control}}

{{DEFAULTSORT:Bridging (Networking)}}

[[Category:Network architecture]]

[[Category:Ethernet]]

[[Category:Networking hardware|Bridge]]