Previous Table of Contents Next


Bridges

Bridges are relatively simple devices that are used to connect LANs of the same architecture (Ethernet-to-Ethernet). Bridges operate at the bottom two layers of the OSI model, providing Physical Layer and Data Link Layer connectivity. Bridges, at the most basic level, act to extend the physical reach of a LAN, passing traffic from one LAN segment to another based on the destination address of the packet. In other words, they act as LAN repeaters where specified distance limitations are exceeded (Figure 9.7). Bridges have buffers, in order that they can store and forward packets in the event that the destination link is congested with traffic.


Figure 9.7  Bridged LAN network.

A key advantage of bridges is their inherent simplicity. As protocol-independent devices, they do not perform complex processes on the data packets traveling through them. Neither do they attempt to evaluate the network as a whole in order to make end-to-end routing decisions. Rather, they read the destination address of the incoming data packet and forward it along its way to the next link. Cascading bridges accomplish this process link-by-link. As a result, bridges are inexpensive and fast. Such bridges can support multiple LANs connected by multiple media. In other words, multiple ports are provided with interfaces to coax, UTP, and fiber optic transmission systems. Additionally multiple LANs of disparate origin can be supported. For instance, Ethernet-to-Ethernet and Token Ring-to-Token Ring connectivity can be provided [9-13]. It also is possible to interconnect disparate LANs such as Ethernet-to-Token Ring, through the use of an encapsulating bridge. Such a bridge encapsulates the native LAN data, surrounding it with control information appropriate to the LAN to which the target device is attached.

More sophisticated bridges add more functionality, although they are more expensive and slower. Such bridges can also route traffic, at a simple level, between LANs based on the destination address. The routing table can be entered into program logic by the system administrator or can be learned by the bridge as it views the originating addresses of traffic passing through it over a period of time [9-14].

Filtering bridges filter traffic that is not addressed to another connected LAN or LAN segment. Filtering bridges are employed in LAN segmentation to reduce unnecessary congestion on LAN segments, improving overall access and throughput.

Media Access Control (MAC) bridges are more sophisticated still. MAC bridges connect unlike, or disparate, LANs (e.g., Ethernet-to-Token Ring). This is accomplished through the process of encapsulation, or translation. When operating in this mode, the bridge alters the packet/frame format, by encapsulating, or enveloping, the original packet with control data specific to the protocol of the destination LAN supporting the target device. Such an approach might be used to connect an Ethernet LAN to a Token Ring LAN, or where 2 Ethernet LANs are connected via a FDDI backbone [9-13].

Specific bridge protocols include Spanning Tree, Source Routing Protocol, and Source Routing Transparent.

Spanning Tree Protocol (STP) bridges
also known as Learning Bridges, are defined in IEEE 802.1 standards. Spanning tree bridges are self-learning, filtering bridges for use in connecting LANs on a point-to-point basis. The bridge is programmed or teaches itself the addresses of all devices on the network; subsequently, the network tree of the bridge provides only one span (link) for each LAN-to-LAN connection. Some spanning tree bridges also have the capability to provide security by denying access to certain resources based on user and terminal ID. Bridges which support the spanning tree algorithm have the ability to automatically reconfigure themselves for alternate paths if a network segment fails, thereby improving overall reliability [9-15].
IBM Source Routing Protocol (SRP)
bridges are programmed with specific routes for each packet, based on considerations such as the physical location of the nodes, and the number of bridges involved. The maximum number of bridges hopped is thirteen (13).
Source Routing Transparent (SRT)
is defined in the IEEE 802.1 standard. It is effectively a combination of STP and SRP. The SRT router can connect LANs by either method, as programmed [9-2].


Previous Table of Contents Next