Bridging

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Bridging is a forwarding technique used in packet-switched computer networks. 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 preclude the need for further broadcasting. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area networks.

Bridging generally refers to Transparent bridging which predominates in Ethernet. Another form of bridging, Source route bridging, was developed for token ring networks.

A Network bridge connects multiple network segments 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 switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with Bridge.

Bridges are similar to repeaters or network hubs, devices that connect network segments at the physical layer; however, with bridging, traffic from one network is managed rather than simply rebroadcast to adjacent network segments. Bridges are more complex than hubs or repeaters. Bridges can analyze incoming data packets to determine if the bridge is able to send the given packet to another segment of the network.

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 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 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 has A's address in its forwarding table so it forwards the response only to host A on 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 Corp. in the 1980s ^[1].

Filtering database

To translate between two segments types, a bridge reads a frame's destination MAC address and decides to either forward or filter. If the bridge determines that the destination node is on another segment on the network, it forwards it (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.

Advantages of network bridges

• Self-configuring
• Simple bridges are inexpensive
• Isolate collision domain
• Reduce the size of collision domain by microsegmentation in non-switched networks
• Transparent to protocols above the MAC layer
• Allows the introduction of management/performance information and access control
• LANs interconnected are separate, and physical constraints such as number of stations, repeaters and segment length don't apply
• Helps minimize bandwidth usage

Disadvantages of network bridges

• Does not limit the scope of broadcasts
• Does not scale to extremely large networks
• Buffering and processing introduces delays
• Bridges are more expensive than repeaters or hubs
• A complex network topology can pose a problem for transparent bridges. For example, multiple paths between transparent bridges and LANs can result in bridge loops. The spanning tree protocol helps to reduce problems with complex topologies.

Bridging versus routing

Bridging and routing 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 addresses while a router makes its decisions according to arbitrarily assigned IP Addresses. 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 domain
• Instrument bridge
• Promiscuous mode
• Spanning Tree Protocol
• Wireless bridge

External links

• Ethernet Bridge for Ethernet Extension use
• Bridging
• Ethernet Bridging (OpenVPN)
• Hamilton, Kenneth; Clark, Kenedy (1999-08-01). "Cisco LAN Switching". Encyclopedia of Things. Cisco Press. http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Bridging-Basics.html. Retrieved 1999-08-01. 

References