Network bridge

A network bridge connects multiple network segments (network domains) at the data link layer. It is sometimes called a network switch, and it works by using bridging. Traffic from one network is forwarded through it to another network. The bridge simply does what its name entails, by connecting two sides from adjacent networks.

A repeater is a similar device that connects network segments at the physical layer. An Ethernet hub is a type of repeater.

Bridging takes place at the data link layer of the OSI model. Therefore a bridge can only read the Ethernet header which provides the MAC address of the source and destination address. When a broadcast packet is transmitted, the bridge floods all the ports with the broadcast packets. Bridges use two methods to resolve the network segment that a MAC address belongs to.

Transparent Bridging – This method uses a forwarding database to send packets across network segments. The forwarding database is initially empty and entries in the database are built as the bridge receives packets. If an address entry is not found in the forwarding table, the packet is flooded to all ports of the bridge which sends the packet to all segments except the source address. This type of bridging is common in Ethernet networks. To avoid frame looping, a spanning tree is created from the network graph and bridges not present in it are kept inactive; they can become active again if another bridge stops working.

Source route bridging – This method is used in Token Ring networks. See below.

In Ethernets, the term "bridge" formally means a device that behaves according to the IEEE 802.1D standard - this is most often referred to as a network switch in marketing literature.

Source route bridging

Two frame types are used in order to find the route, SR (Single-Route) frame (most of the traffic, they specify the route they'll follow) and AR (All-Routes) frame (used to find routes). AR frames are sent by flooding on all network branches; each step of the followed route is registered by the bridge performing it. To avoid indefinite looping of AR frames, they have a maximum hop count (which must be greater than the diameter of the network graph) decremented by each router and are dropped when it reaches zero. The first copy of an AR frame reaching the destination is considered to have followed the best route, and the route can be used for subsequent SR frames; the other copies are discarded. This allows indirect load balancing among multiple bridges connecting two networks. In fact, the more a bridge is loaded, the more it is unlikely to take part in the best route for a new destination, since it will be slow in packet forwarding; so a new AR packet will find a different route. This is very different from transparent bridge usage, where redundant bridges will be inactivated; however, more overhead is introduced to find routes, and space is wasted to store them in packets. A switch with a faster backplane can be just as good for performances (not for fault tolerance).

Advantages of Network bridge

Self configuring
Primitive bridges are very cheap
Increase collision domain
Transparent to protocols above the MAC layer
Allows the introduction of management - performance information and access control
LANs interconnected are separate and physical contraints such as number of stations, repeaters and segment length do apply

Disadvantages of a Network bridge

Does not limit the scope of broadcasts
Does not scale to extremely large networks
Buffering introduces store and forward delays - on average traffic destined for bridge will be related to the number of stations on the rest of the LAN
Bridging of different MAC protocols introduces errors

Bridging VS Routing

Bridging and Routing are very different, bridging refers to _Data_Link_Layer (Data-Link Layer) while Routing refers to _Network_Layer (Network Layer). The main difference is that a bridge routes according to MAC Addresses while a router according to IP Addresses. As a result of this, bridges are unable to distinguish networks while routers can. Bridges are very similar to Switches.

This is mainly a discussion people have when designing a network. You can choose to put multiple segments into one bridged network, or to divide it into different networks interconnected by routers. If a host moves between segments in a routed network, it has to get a new IP address, and thereby break all existing TCP connections; if it moves in a bridged network, it doesn't have to reconfigure anything. Bridging is also a cheaper option when you need to separate broadcast domains as bridges are significantly cheaper than routers.

Specific uses of the term "bridge"

A description of the Network Bridge in Windows XP is given here - it allows a Windows XP system to function as a bridge between the various networking devices connecting to it.

Documentation on Linux bridging can be found in the Linux networking wiki in here. Linux bridging allows filtering and routing.

External links

Bridging