Bridging 101

Note: This is a very brief introduction to the concept of bridging networks. For more in--depth information, there are a variety of references, including the IEEE 802.3d spanning tree specification. A good general purpose reference is Radia Perlman's "Interconnections" (Addison-Wesley Publishing).

Bridging Basics

Bridges are used to limit the amount of traffic appearing on network segments other than the destination segment. They do not provide for the logical grouping of network devices, which makes them considerably less flexible than routers from the standpoint of network management.

As opposed to routers, bridges operates on the "physical" network layer. While protocols such as IP or IPX are concerned with their own addressing schemes and routing tables, bridging is only concerned with physical (i.e. Ethernet) addresses, and which bridge port they are located on. This simplicity is both a strength of bridging and a weakness. Because bridges maintain very little information about network topology, they are easier to configure than routers. But for this same reason, they do not limit traffic on network segments as well as routers do, and they are more prone to propagating network problems from one segment to another.

Note: Ethernet switches are actually just a new name for multiport bridges.

Transparent Bridging

The simplest kind of bridge is called a transparent bridge. It operates according to the following rules:

• Examine all packets on all active network ports for their source address.
• Maintain a table that tracks which port a source address has appeared on.
• Look up the destination addresses in this table for all packets, and if a packet's matching port is different than the port it was received on, forward the packet to the matching port.
• If no match is found, or if the destination address is the broadcast address, forward the packet out all active ports.

This scheme is acceptable on very simple network topologies. It will not work correctly if there are multiple paths to the same destination. In this case, packets will be forwarded in a "bridging loop" which will quickly use up the available network bandwidth on the segments in question.

Spanning Tree Bridging

To avoid bridging loops, an algorithm was developed which lets bridges shut off ports which provide duplicate paths to the same destination. This "spanning tree" algorithm was ratified as an IEEE standard (802.1d), and is supported by most bridge/switch vendors. The algorithm relies on the use of Bridge Protocol Data Units (BPDU packets), which provide information to all bridges about the "distance" in hops to each bridge port from a "root bridge." The root bridge is selected using settings entered into each bridge (with the Ethernet address acting as a tie-breaker).

Using BPDU information, a bridge can determine whether one of its ports provides an optimal path to the root bridge. If it does not, the port is shut down. If the path distance is optimal but is the same as another bridge's path, a simple protocol allows one of the ports to be shut down.

In all other respects, spanning tree bridges operate in the same fashion as simple learning bridges.