Bridge Placement and Bridging Loops

There are two issues that you must consider when using bridges. The first is the bridge placement, and the other is the elimination of bridging loops:

  • Placement Bridges should be positioned in the network using the 80/20 rule. This rule dictates that 80% of the data should be local and that the other 20% should be destined for devices on the other side of the bridge.

  • Bridging loops Bridging loops can occur when more than one bridge is implemented on the network. In this scenario, the bridges can confuse each other by leading one another to believe that a device is located on a certain segment when it is not. To combat the bridging loop problem, the IEEE 802.1d Spanning Tree protocol enables bridge interfaces to be assigned a value that is then used to control the bridge-learning process.

Types of Bridges

Three types of bridges are used in networks:

  • Transparent bridge Derives its name from the fact that the devices on the network are unaware of its existence. A transparent bridge does nothing except block or forward data based on the MAC address.

  • Source route bridge Used in Token Ring networks. The source route bridge derives its name from the fact that the entire path that the packet is to take through the network is embedded within the packet.

  • Translational bridge Used to convert one networking data format to another; for example, from Token Ring to Ethernet and vice versa.

Today, bridges are slowly but surely falling out of favor. Ethernet switches offer similar functionality; they can provide logical divisions, or segments, in the network. In fact, switches are sometimes referred to as multiport bridges because of the way they operate.