Bridges

 

Bridges – What is Bridges? Bridge Protocols

Bridges are connection devices between networks that operate in the data link layer of the OSI model. It means that bridges have more functionality (in terms of connection) than Layer 1 devices, such as repeaters and hubs. Bridges are used to segment networks that have grown to a point where data traffic through the physical environment of the network slows down the global transfer of information.

Transparent bridges build a routing table.

Transparent bridges are used in Ethernet networks to forward packets (and isolate those that are part of the local segment traffic) in the network according to a routing table. The bridge constructs this table by creating samples of the packets received at its different ports until a complete list of the MAC addresses of the network and the particular network segment in which they located obtained.

Bridges with the routing of origin

Bridges with source routing in Token Ring networks do not offer as many features as transparent bridges in Ethernet networks. Bridges with source routing receive packets that specify the route that those packets should follow. The bridge has to read the address that each package includes to re-issue it to the corresponding segment.

Media Access Control (MAC) Bridge

This is used to connect dissimilar LANs such as Ethernet -to-token ring using encapsulation or translation. This bridge translates the original’ packet format from the requesting LAN segment by encapsulating or enveloping with control data specific to the protocol of the destination LAN segment.

Address Table

As explained above, each bridge should have an address table that indicates the location of different computers or nodes on the segments of LAN. More specifically, it indicates the connection between nodes and ports. When a bridge is booted first time, this table is found to be blank. Now, this question arises how this table is filled with appropriate addresses of different nodes attached to ports. Most of the bridges are called adaptive or self-leaning bridges because they learn the location of the node and associated port themselves and make a list of nodes attached to each segment.

Bridge Protocols

Bridge protocols include spanning tree, source routing protocol, and source routing transparent.

Spanning Tree Protocol (STP) Bridge

This is also known as adaptive or self-learning bridges and is defined in IEEE 802.1 standards. It has already been explained in the above section. Ideally, in bridged network, the network tree of the bridge provides only one span (link) for each LAN-to-LAN connection and therefore, no network with bridges can form a loop. Sometimes, looping can occur.

IBM Source Routing Protocol (SRP) Bridge

These are programmed with specific routes for each packet based on considerations such as the physical location of the nodes, and the number of bridges involved.

Source Routing Transparent (SRT)

It is defined in theIEEE802.1 standard. It is effectively a combination of STP and SRP. The SRT router can connect LANs by either method, as programmed.

Classification of Bridges

These are classified into local and remote bridges:

1. Local bridges are ordinary bridges

2. Remote bridges are used to connect networks that are far from each other. A WAN is generally provided between two bridges