Bridges

Bridges were originally designed to interconnect Ethernet segments together. Most bridges today support filtering and forwarding, as well as Spanning Tree Algorithm. The IEEE 802.1D specification is the standard for bridges.

During initialization, the bridges learns about the network and the routes. Packets are passed onto other network segments based on the MAC layer. Each time the bridge is presented with a fame, the source address is stored. The bridge builds up a table which identifies the segment to which the device is located on. This internal table is then used to determine which segment incoming frames should be forwarded to. The size of this table is important, especially if the network has a large number of workstations/servers.

The advantages of bridges are:

• increase the number of attached workstations and network segments.
• since bridges buffer frames, it is possible to interconnect different segments which use different MAC protocols.
• since bridges work at the MAC layer, they are transparent to higher level protocols.
• by subdividing the LAN into smaller segments, this increases overall reliability, and makes the network easier to maintain.

The disadvantages of bridges are

• the buffering of frames introduces network delays.
• bridges may overload during periods of high traffic.
• bridges which combine different MAC protocols require the frames to be modified before transmission onto the new segment. This causes delays.

Transparent bridges (also known as spanning tree IEEE 802.1D) make all routing decisions. The bridge is said to be transparent (invisible) to the workstations. The bridge will automatically initialize itself and configure its own routing information after it has been enabled.

Bridges are ideally used in environments where there a number of well defined workgroups, each operating more or less independent of each other, with occasional access to servers outside of their localized workgroup or network segment. Bridges do not offer performance improvements when used in diverse or scattered workgroups, where the majority of access occurs outside of the local segments.

The two separate network segments can be connected via a bridge. Note that each segment must have a unique network address number in order for the bridge to be able to forward packets from one segment to the other.

Ideally, if workstations on network segment A needed access to a server, the best place to locate that server is on the same segment as the workstations, as this minimizes traffic on the other segment and avoids the delay incurred by the bridge.