Local Area Network Bridges Essay, Research Paper

Local Area Network Bridges

Both the physical distance that Local Area Network (LAN) can cover and the number of hosts that can be attached to it are limited. To overcome this limitation, bridges are introduced as devices which connect LANs at the MAC layer. The purpose of bridges is to allow hosts attached to different LANs to communicate as if they were located on the same LAN. In contrast to repeaters, that act at the physical layer and allow all traffic to cross LAN segments, bridges are more intelligent and limit the traffic to the section of the network on which it is relevant. To accomplish this, bridges must make a forwarding decision with each received frame regarding where to send the frame so that it reaches its destination. There are two different bridge standards: Source-Routing (SR), which is common in Token Ring environment, and Transparent Spanning Tree (TST), which common in Ethernet environment. The IEEE standard for MAC bridges is ANSI/IEEE 802.1D: MAC Sub-layer Interconnection.

Bridge Routing Requirements

In general, LANs are low-cost, low-delay, high-bandwidth (e.g., 1-10 Mbps) broadcast channels. A bridged LAN environment preserves the low-delay and high-bandwidth feature but its topology may be more dynamic than in a single LAN due to possible bridge or LAN failures and hosts being moved around.

Bridge routing algorithms should meet the following requirements:

1) A bridges LAN environment should resemble a single LAN environment as closely as possible. In other words, the extension should be transparent to hosts.

2) The transparency requirement extends to performance requirements, such as low transmission delays, low undetected data corruption and FIFO maintenance.

3) Bridges routing algorithms should be able to adapt quickly to environment changes.

Source Routing Bridges

Source Routing is mostly used to interconnect token ring LANs. In Source-Routing, the source station must determine, in advance, the route to the LAN of the destination station, and include this route in the header of each frame. To determine the routing information, the source stations first issues a search frame, which is general an LCC TEST commands on its ring. If a response is received from the desired destination station, it indicates that both source and destination stations are on the same ring and that no routing information is required. However, if no respond is received, the source station issues a route discovery frame, which fans-out on every ring in the LAN segment. As the frame is forwarded from one ring to another, each bridge updates the routing information in the search frame. When the search frame reaches the destination, it contains the route between the source and destination station. The destination station then sends a response frame to the source station, with the routing information. Both station then use the routing information in each subsequent sent to each other.

Source Routing uses two key parameters to identify a route between a source station and a destination station. These parameters are ring numbers and bridge numbers. Each ring is assigned a unique number. These numbers generally range between 1 and FFF (hex). Each bridge is assigned a bridge numbers, ranging from 0 to F (hex). The only restriction when assigning bridge number is that parallel bridges, connecting identical rings, must have different bridge numbers. The route between the source and the destination stations consists of LAN numbers and bridge numbers. The route is obtained by each bridge which received the route discovery from adds to the existing route, its number and the ring number that it forwards this frame to.

Transparent Spanning Tree Bridges

In Transparent Spanning Tree, the TST bridges create a spanning tree. Routing information is not needed to be determined by the source station, and all the routing is done by the bridges. Therefore, stations from different LANs connected with TST-bridges can communicate as though both stations are on the same LAN. By maintaining a tree structure routing become very simple and dynamically maintain a station address database, referred to as a forwarding database. Each entry in the database has a station, age, and port identifier. The port identifier is used to identify the bridge port on which the bridge should forward frames destined to the station. The forwarding database is built by observing frame that passes though the bridge, and learning from them the station’s location. Thus a TST-bridge has two function: learning and forwarding.

a) Bridge Learning

The source address of all frame received is compare against the information in the forwarding database. If the source address is not found in the forwarding database, it is added along with the port identifier it received on. The age value of this database entry is reset to indicate that this is a “fresh” entry. If the source address already exists in the forwarding database, and the database indicates that the station was last seen on a different bridge port, then the port identifier for the entry is changes to the port identifier on which the frame was received, and the age value of this entry is reset. If the frame is received on the same port as the port in the database entry, then only the age value of this entry is reset.

b) Bridge forwarding

When a frame is received is on a bridge port, the destination address contained in the frame header is compared to the information contained in the forwarding database. If the destination address is not found in the forwarding database, the frame is transmitted on all bridge ports, except the one on which it was received. If the destination address is found in the forwarding database, and the port identifier kept along with this address is the same as the identifier of the bridge port on which that frame was received, the frame is not forwarded to any bridge port. If the port identifiers are different, the frame is forwarded on the bridge port identified in the forwarding database.

References

Interconnections: Bridges and Routers, Radia Perlman, Addison Wesley, May 1992, pg. 225-250

Local Area Networks: An Introduction to the Technology, John E. McNamara Butterworth-Heinemann, March 1997 , pg. 106-122