Bridging (networking)

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A high-level overview of network bridging, using the ISO/OSI layers and terminology

A network bridge is a computer networking device that creates a single aggregate network from multiple communication networks or network segments. This function is called network bridging.[1] 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.[2] In the OSI model, bridging is performed in the data link layer (layer 2).[3] If one or more segments of the bridged network are wireless, the device is known as a wireless bridge.

There are four main types of network bridging technologies: simple bridging, multiport bridging, learning or transparent bridging, and source route bridging.[4][5]

Transparent bridging[edit]

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 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 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 (DEC) originally developed the technology in the 1980s.[6]

In the context of a two-port bridge, one can think of the forwarding information base as a filtering database. A bridge reads a 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.

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 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 in network.

Simple bridging[edit]

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.

Multiport bridging[edit]

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 switches.

Implementation[edit]

The forwarding information base stored in content-addressable memory (CAM) is initially empty. For each received ethernet frame the switch learns from the frames source MAC address and adds this together with the ingress interface to build the forwarding information base. The switch then forwards the frame to the interface found in the CAM based on the frames destination MAC address. If the destination address is unknown the switch sends the frame out on all interfaces (except ingress interface). This behaviour is called unicast flooding.

Forwarding[edit]

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:

  1. Store and forward: the switch buffers and verifies each frame before forwarding it; a frame is received in its entirety before it is forwarded.
  2. 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.
  3. Fragment free: a method that attempts to retain the benefits of both store and forward and cut through. Fragment free checks the first 64 bytes 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 frames that are in error because of a collision will not be forwarded. This way the frame will always reach its intended destination. Error checking of the actual data in the packet is left for the end device.
  4. Adaptive switching: a method of automatically selecting between the other three modes.[7][8]

Shortest Path Bridging[edit]

Shortest Path Bridging (SPB), specified in the IEEE 802.1aq standard, is a computer networking technology intended to simplify the creation and configuration of networks, while enabling multipath routing.[9][10][11]

It is the replacement for the older spanning tree protocols: IEEE 802.1D, IEEE 802.1w, IEEE 802.1s. These blocked any redundant paths that could result in a layer 2 loop, whereas SPB allows all paths to be active with multiple equal cost paths, provides much larger layer 2 topologies,[12] supports faster convergence times, and improves the efficiency by allowing traffic to load share across all paths of a mesh network.[13][14][15][16] It is designed to virtually eliminate human error during configuration and preserves the plug-and-play nature that established Ethernet as the de facto protocol at layer 2.

The technology provides logical Ethernet networks on native Ethernet infrastructures using a link state protocol to advertise both topology and logical network membership. Packets are encapsulated at the edge either in media access control-in-media access control (MAC-in-MAC) 802.1ah or tagged 802.1Q/802.1ad frames and transported only to other members of the logical network. Unicast, multicast, and broadcast are supported and all routing is on symmetric shortest paths.

The control plane is based on the Intermediate System to Intermediate System (IS-IS), leveraging a small number of extensions defined in RFC 6329.[17]

See also[edit]

References[edit]

  1. ^ "Traffic regulators: Network interfaces, hubs, switches, bridges, routers, and firewalls" (PDF). Cisco Systems. 1999-09-14. Retrieved 2012-07-27.
  2. ^ "What is a Network Switch vs. a Router?". Cisco Systems. Retrieved 2012-07-27.
  3. ^ "RFC 1286 - Definitions of Managed Objects for Bridges". Tools.ietf.org. 1989-07-14. Retrieved 2013-10-19.
  4. ^ "Local Area Networks: Internetworking". manipalitdubai.com. Archived from the original (PPT) on 2014-05-13. Retrieved 2012-12-02.
  5. ^ "Bridging Protocols Overview" (PPT). iol.unh.edu. Retrieved 2012-12-02.
  6. ^ "Transparent Bridging". Cisco Systems, Inc. Retrieved 2010-06-20.
  7. ^ Dong, Jielin. Network Dictionary. Javvin Technologies Inc. p. 23. ISBN 9781602670006. Retrieved 25 June 2016.
  8. ^ "Cray makes its Ethernet switches responsive to net conditions". IDG Network World Inc. 1 July 1996. Retrieved 25 June 2016.
  9. ^ "Alcatel-Lucent, Avaya, Huawei, Solana and Spirent Showcase Shortest Path Bridging Interoperability". Huawei. 7 September 2011. Retrieved 11 September 2011.
  10. ^ An improved shortest path bridging protocol for Ethernet backbone network. IEEE Xplore. 3 March 2011. doi:10.1109/ICOIN.2011.5723169. ISBN 978-1-61284-661-3. ISSN 1976-7684. Retrieved 11 September 2011.
  11. ^ "Lab Testing Summary Report; Data Center Configuration with SPB" (PDF). Miercom. September 2011. Retrieved 25 December 2011.
  12. ^ Shuang Yu. "IEEE approves new IEEE 802.1aq™ Shortest path bridging". IEEE Standards Association. Retrieved 19 June 2012. 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.
  13. ^ Peter Ashwood-Smith (24 February 2011). "Shortest Path Bridging IEEE 802.1aq Overview" (PDF). Huawei. Archived from the original (PDF) on 15 May 2013. Retrieved 11 May 2012.
  14. ^ Jim Duffy (11 May 2012). "Largest Illinois healthcare system uproots Cisco to build $40M private cloud". PC Advisor. Retrieved 11 May 2012. Shortest Path Bridging will replace Spanning Tree in the Ethernet fabric.
  15. ^ "IEEE Approves New IEEE 802.1aq Shortest Path Bridging Standard". Tech Power Up. 7 May 2012. Retrieved 11 May 2012.
  16. ^ D. Fedyk, Ed.,; P. Ashwood-Smith, Ed.,; D. Allan, A. Bragg,; P. Unbehagen (April 2012). "IS-IS Extensions Supporting IEEE 802.1aq". IETF. Retrieved 12 May 2012.CS1 maint: Multiple names: authors list (link)
  17. ^ "IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging". IETF. April 2012. Retrieved 2 April 2012.

Further reading[edit]