Title - Bridge Technologies


Bridges are devices that pass packets between two LANs. The biggest benefit of a bridge is to create smaller networks which can greatly increase the overall performance. In a LAN sharing a common wire, when one computer is talking all others must be listening. If a network of 100 computers is broken into two networks of 50 computers the overall bandwidth is doubled. The bridge connecting these two LAN segments allows a computer on one LAN to talk to a computer on the other LAN.


Bridges operate on the OSI layer 2 (i.e.. Ethernet) and they are often confused with routers that operate on the OSI layer 3. There are several types of bridges commonly used and each type has advantages and disadvantages. The basic types of bridging algorithms are transparent, spanning tree, source-routing, and source-routing transparent.


The Institute of Electrical and Electronic Engineers (IEEE) has divided the OSI link layer into two separate sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. The MAC sublayer permits and orchestrates media access (for example, contention, token passing, or others), while the LLC sublayer is concerned with framing, flow control, error control, and MAC-sublayer addressing.

Some bridges are MAC-layer bridges. These devices bridge between homogeneous networks (for example, IEEE 802.3 and IEEE 802.3). Other bridges can translate between different link-layer protocols (for example, IEEE 802.3 and IEEE 802.5). A bridge's translation between networks of different types is never perfect because it is likely that one network will support certain frame fields and protocol functions not supported by the other network.

Transparent Bridging

The simplest kind of bridge is called a transparent bridge. Transparent bridges are so named because their presence and operation are transparent to network hosts. When transparent bridges are powered on, they learn the network's topology by analyzing the source address of incoming frames from all attached networks. If, for example, a bridge sees a frame arrive on line 1 from Host A, the bridge concludes that Host A can be reached through the network connected to line 1. Through this process, transparent bridges build a table which is the basis for traffic forwarding.

Transparent bridges successfully isolate intrasegment traffic, thereby reducing the traffic seen on each individual segment. This usually improves network response times as seen by the user. The extent to which traffic is reduced and response times are improved depends on the volume of intersegment traffic relative to the total traffic as well as the volume of broadcast and multicast traffic.


Transparent Bridges operate according to the following rules:

1. Examine all packets on all active network ports for their source address.
2. Maintain a table that tracks which port a source address has appeared on.
3. 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.
4. 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.

Bridging Loops

Without a bridge-to-bridge protocol, the transparent bridge algorithm fails when there are multiple paths of bridges and local-area networks (LANs) between any two LANs in the internetwork. Figure 1 illustrates such a bridging loop.

Figure 1. Inaccurate Forwarding and Learning in Transparent Bridging Environments

Suppose Host A sends a frame to Host B. Both bridges receive the frame and correctly conclude that Host A is on Network 2. Unfortunately, after Host B receives two copies of Host A's frame, both bridges will again receive the frame on their network 1 interfaces because all hosts receive all messages on broadcast LANs. In some cases, the bridges will then change their internal tables to indicate that Host A is on network 1. If so, when Host B replies to Host A's frame, both bridges will receive and subsequently drop the replies because their tables will indicate that the destination (Host A) is on the same network segment as the frame's source.

In addition to basic connectivity problems such as the one just described, the proliferation of broadcast messages in networks with loops represents a potentially serious network problem. Referring again to Figure 1, assume that Host A's initial frame is a broadcast. Both bridges will forward the frames endlessly, using all available network bandwidth and blocking the transmission of other packets on both segments.

A topology with loops such as that shown in Figure 1 can be useful as well as potentially harmful. A loop implies the existence of multiple paths through the internetwork. A network with multiple paths from source to destination can increase overall network fault tolerance through improved topological flexibility.


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. 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.

Spanning-Tree Algorithm (STA)

The spanning-tree algorithm (STA) was developed to preserve the benefits of loops while eliminating their problems. The STA designates a loop-free subset of the network's topology by placing those bridge ports that, if active, would create loops into a standby (blocking) condition. Blocking bridge ports can be activated in the event of primary link failure, providing a new path through the internetwork.


The STA uses a conclusion from graph theory as a basis for constructing a loop-free subset of the network's topology. Graph theory states the following:


"For any connected graph consisting of nodes and edges connecting pairs of nodes, there is a spanning tree of edges that maintains the connectivity of the graph but contains no loops."


Figure 2 illustrates how the STA eliminates loops. The STA calls for each bridge to be assigned a unique identifier. Typically, this identifier is one of the bridge's Media Access Control (MAC) addresses plus a priority. Each port in every bridge is also assigned a unique (within that bridge) identifier (typically, its own MAC address). Finally, each bridge port is associated with a path cost. The path cost represents the cost of transmitting a frame onto a LAN through that port. In Figure 2, path costs are noted on the lines emanating from each bridge. Path costs are usually defaulted, but can be assigned manually by network administrators.

Figure 2. Transparent Bridge Network before Running STA

The first activity in spanning-tree computation is the selection of the root bridge, which is the bridge with the lowest value bridge identifier. In Figure 2, the root bridge is Bridge 1. Next, the root port on all other bridges is determined. A bridge's root port is the port through which the root bridge can be reached with the least aggregate path cost. This value (the least aggregate path cost to the root) is called the root path cost.

Finally, designated bridges and their designated ports are determined. A designated bridge is the bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge. A LAN's designated port is the port that connects it to the designated bridge.

In some cases, two or more bridges can have the same root path cost. For example, in Figure 2, Bridges 4 and 5 can both reach Bridge 1 (the root bridge) with a path cost of 10. In this case, the bridge identifiers are used again, this time to determine the designated bridges. Bridge 4's LAN V port is selected over Bridge 5's LAN V port.

Using this process, all but one of the bridges directly connected to each LAN are eliminated, thereby removing all two-LAN loops. The STA also eliminates loops involving more than two LANs, while still preserving connectivity. Figure 28 shows the results of applying the STA to the network shown in Figure 2. Figure 3 shows the tree topology more clearly. Comparing this figure to the pre-spanning-tree figure shows that the STA has placed both Bridge 3's and Bridge 5's ports to LAN V in standby mode.

Figure 3. Transparent Bridge Network After Running STA

The spanning-tree calculation occurs when the bridge is powered up and whenever a topology change is detected. The calculation requires communication between the spanning-tree bridges, which is accomplished through configuration messages (sometimes called bridge protocol data units, or BPDUs). Configuration messages contain information identifying the bridge that is presumed to be the root (root identifier) and the distance from the sending bridge to the root bridge (root path cost). Configuration messages also contain the bridge and port identifier of the sending bridge and the age of information contained in the configuration message.

Bridges exchange configuration messages at regular intervals (typically one to four seconds). If a bridge fails (causing a topology change), neighboring bridges will soon detect the lack of configuration messages and initiate a spanning-tree recalculation.

All transparent bridge topology decisions are made locally. Configuration messages are exchanged between neighboring bridges. There is no central authority on network topology or administration.

Source-Route Bridging

The source-route bridging (SRB) algorithm was developed by IBM and proposed to the IEEE 802.5 committee as the means to bridge between all local-area networks (LANs). Subsequently, IBM has offered a new bridging standard to the IEEE 802 committee: the source-route transparent (SRT) bridging solution. SRT bridging eliminates pure SRBs entirely, proposing that the two types of LAN bridges be transparent bridges and SRT bridges. Although SRT bridging has achieved support, SRBs are still widely deployed.

SRB Algorithm

SRBs are so named because they assume that the complete source-to-destination route is placed in all inter-LAN frames sent by the source. SRBs store and forward the frames as indicated by the route appearing in the appropriate frame field. Figure 4 illustrates a sample SRB network.

Figure 4. SRB Network Diagram

Referring to Figure 4, assume that Host X wishes to send a frame to Host Y. Initially, Host X does not know whether Host Y resides on the same or a different LAN. To determine this, Host X sends out a test frame. If that frame returns to Host X without a positive indication that Host Y has seen it, Host X must assume that Host Y is on a remote segment.

To determine the exact remote location of Host Y, Host X sends an explorer frame. Each bridge receiving the explorer frame (Bridges 1 and 2 in this example) copies the frame onto all outbound ports. Route information is added to the explorer frames as they travel through the internetwork. When Host X's explorer frames reach Host Y, Host Y replies to each individually using the accumulated route information. Upon receipt of all response frames, Host X chooses a path based on some predetermined criteria.


In the example in Figure 4, this process will yield two routes:

  • LAN 1 to Bridge 1 to LAN 3 to Bridge 3 to LAN 2

  • LAN 1 to Bridge 2 to LAN 4 to Bridge 4 to LAN 2

Host X must select one of these two routes. The IEEE 802.5 specification does not mandate the criteria Host X should use in choosing a route, but it does make several suggestions, including the following:

  • First frame received

  • Response with the minimum number of hops

  • Response with the largest allowed frame size

  • Various combinations of the above criteria

In most cases, the path contained in the first frame received will be used.
After a route is selected, it is inserted into frames destined for Host Y in the form of a routing information field (RIF). A RIF is included only in those frames destined for other LANs. The presence of routing information within the frame is indicated by the setting of the most significant bit within the source address field, called the routing information indicator (RII) bit.

Source-Route Transparent Bridging

Source-Route Transparent (ART) bridges combine implementations of the transparent bridging and the Source Routing bridging algorithms. ART bridges use the Routing Information Indicator (RII) bit to distinguish between frames employing FRB and frames employing transparent bridging. If the RII bit is 1, a Routing Information Frame (RIF) is present in the frame, and the bridge uses the SRB algorithm. If the RII bit is 0, an RIF is not present, and the bridge uses transparent bridging.
Like translational bridges, SRT bridges are not perfect solutions to the problems of mixed-media bridging. SRT bridges must still deal with the Ethernet/Token Ring incompatibilities described earlier. SRT bridging is likely to require hardware upgrades to SRBs to allow them to handle the increased burden of analyzing every packet. Software upgrades to SRBs may also be required. Further, in environments of mixed SRT bridges, transparent bridges, and SRBs, source routes chosen must traverse whatever SRT bridges and SRBs are available. The resulting paths can potentially be substantially inferior to spanning-tree paths created by transparent bridges. Finally, mixed SRB/SRT bridging networks lose the benefits of SRT bridging, so users will feel compelled to execute a complete cutover to SRT bridging at considerable expense. Still, SRT bridging permits the coexistence of two incompatible environments and allows communication between SRB and transparent bridging end nodes.


In summary, the four major bridging algorithms in use today are:

  • Transparent Bridging (TB)

    • Examines source/destination packet addressess to determine if it forwards packet

  • Spanning Tree Bridging

    • Finds root bridge and determines hiearchy of bridges

    • Finds multiple paths and shuts down duplicates

  • Source-Routed Bridge (SRB)

    • Initiator finds path to destinate (through All Routes Explore message)

    • Puts bridge IDSs into each packet header

  • Source-Routed Transparent Bridge (SRTB)

    • Detect best bridging method available


2018 NextGen Datacom, Inc.