Networking Devices - Bridge

Bridge – A bridge operates at data link layer. A bridge is a repeater, with add on functionality of filtering content by reading the MAC addresses of source and destination.

Bridges can examine MAC addresses (also called hardware addresses or physical addresses) in each data packet that circulates through the segments of the network that connects the bridge. By knowing which MAC addresses reside in each of the segments of the network, the bridge can prevent data traffic from a specific segment from passing to another segment of the network that also connected to the bridge.

It passes information from one LAN segment to another based on the destination address of the packet. When a bridge receives data through one of its ports, it checks the data for a MAC address. If this address matches that of the node connected to other port, the bridge sends this data through this port. This action is called forwarding. If the address does not match with any node connected to other port, the bridge discards it. This action is called filtering.

Unlike repeaters, bridges have buffers to store and forward packets in the event that the destination link is congested with traffic.

It is also used for interconnecting two LANs working on the same protocol. It has a single input and single output port.

Types of Bridges:

Transparent Bridges: - A transparent bridge is a common type of bridge that observes incoming network traffic to identify media access control (MAC) addresses. These bridges operate in a way that is transparent to all the network's connected hosts. A transparent bridge records MAC addresses in a table that is much like a routing table and evaluates that information whenever a packet is routed toward its location. A transparent bridge may also combine several different bridges to better inspect incoming traffic. Transparent bridges are implemented primarily in Ethernet networks.

Source Routing Bridges: - Source routing bridge decides the route between two hosts. Source routing bridge uses the MAC destination address of a frame to direct it by the source routing algorithm. In source routing, the route over which the frame is to send is known to every station on the extended LAN. The routing information is stored in the frames. 

Difference between Transparent Bridge and Source Routing Bridge:

S.NO.

TRANSPARENT BRIDGE

SOURCE ROUTING BRIDGE

1.	Transparent bridge service is connectionless.	Source Routing Bridge service is connection oriented.
2.	In transparent bridge mechanism bridges automatically develop a routing table.	In source routing bridge, bridges do not maintain any routing information.
3.	Transparent bridge does not support multipath routing.	Source routing bridge can make use of multiple path to same destination.
4.	The path used by transparent bridge between any two hosts may not be the optimal path.	Source route bridge always uses the optimal path.
5.	Failures are handled by the transparent bridge on its own.	Host handles the failure of bridge on its own.
6.	Transparent bridges are fully transparent to the users.	Source routing bridges are not visible to the hosts.
7.	The frame processing delay is more.	The frame processing delay is less.
8.	Load sharing is not possible through blocked routes.	Load sharing is possible by judicious choice of routes.