Ethernet Bridges

Bridges were used to break-up the collision domain when 10 Mbps Ethernet was used. The CSMA/CD bus arbitration protocol shared the bandwidth among all users on the LAN segment. With 100 Mbps and faster Ethernet, Ethernet switches are used which don't share bandwidth and provide a circuit switched network. So why learn about bridges? Bridges are the foundation for Ethernet switches. Another name for an Ethernet switch is a multi-port bridge.

Bridges are both hardware and software devices. They can be standalone devices - separate boxes specifically designed for bridging applications, or they can be dedicated PCs with 2 NICs and bridging software. Most servers software will automatically act as a bridge when a second NIC card is installed.

Bridge OSI Operating Layer

Bridges operate on the OSI Model Data Link Layer. They look at the MAC addresses for Ethernet and Token Ring to determine whether or not to forward or ignore a frame.

Purpose of a Bridge

The purposes of a Bridge are:

  • Isolates networks by MAC addresses
  • Manages network traffic by filtering frames
  • Translate from one protocol to another

Isolates networks by MAC addresses

For example, you have 1 segment called Segment 100 with 50 users in several departments using this network segment. The Engineering Dept. is CAD (Computer Aided Design) oriented and the Accounting Dept. is into heavy number crunching: year end reports, month end statements etc..

On this network, any traffic between Client A, B or C and the Accounting File Server in the Accounting Dept. will be heard across the Segment 100. Likewise any traffic between the Engineering Dept.'s Clients G, H or I to the CAD File Server will be heard throughout the Network Segment. The result is that the "Other" Departments access to the Generic File Server is incredibly slow because of the unnecessary traffic occurring due to other departments: Engineering & Accounting.

Note: The designations A, B, and C are used instead of MAC addresses for brevity. The actual MAC addresses would be hexadecimal numbers such as 08-00-EF-45-DC-01.

The solution is to use a Bridge to isolate the Accounting Dept. and another bridge to isolate the Engineering Department. The Bridges will only allow frames to pass through that are not on the local segment. The bridge will first check its "bridging" table to see if the frame is on the local segment, if it is, it will ignore the frame and not forward it to the remote segment. If Client A sent a frame to the Accounting File Server, Bridge #1 will check its bridging table, to see if the Accounting File Server is on the local port. If it is on the local port, Bridge #1 will not forward the frame to the other segments.

If Client A sent a frame to the Generic File Server, again Bridge #1 will check its bridging table to see if the Generic File Server is on the local port. If it is not, then Bridge #1 will forward the frame to the remote port.

Note: The terms local and remote ports are abitrarily chosen to distinguish between the two network ports available on a bridge.

In this manner the network is segmented and the local department traffic is isolated from the rest of the network. Overall network bandwidth increases because the Accounting Dept. does not have to fight with the Engineering Dept. for access to the segment. Each segment has reduced the amount of traffic on it and the result is faster access. Each department still has complete access to the other segments but only when required.

Manages network traffic by filtering frames

Bridges listen to the network traffic and build an image of the network on each side of the bridge. This image of the network indicates the location of each node and the bridge's port that accesses it. With this information, a bridge can make a decision whether to forward the frame across the bridge if the destination address is not on the same port or it can decide to not forward the frame if the destination is on the same port.

This process of deciding whether or not to forward a frame is termed filtering frames. Network traffic is managed by deciding which frames can pass through the bridge. The bridge filters frames.

Translate from one protocol to another

The MAC layer also contains the bus arbitration method used by the network. This can be CSMA/CD as used in Ethernet or Token Passing as used in Token Ring. Bridges are aware of the Bus Arbitration and special translation bridges can be used to translate between Ethernet and Token Ring.

Bridge Segment to Segment Characteristics

Bridges physically separate a network segments by managing the traffic based on the MAC address.

Bridges are store and forward devices. They receive a frame on the local segment, store it and wait for the remote segment's to be clear before forwarding the frame.

There are 2 physical types of bridges: Local and Remote Bridges.

Local Bridges are used as in the previous examples where the network is being locally (talking physical location now) segmented. The 2 segments are physically close together: same building, same floor, etc... Only 1 bridge is required.

Remote Bridges are used in pairs and where the network is remotely segmented (again talking physical locations). The 2 segments are physically far apart: different buildings, different floors, etc... 2 x Half Bridges are required: one at each segment. The Remote bridges are 1/2 of a normal bridge and may use several different communications media inbetween.

Continue on to Ethernet Bridges - Page 2

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Copyright July 2013 Eugene Blanchard