Comparison of Two Bridge Routing Approaches Lixia Zhang Introduction 0th the physical distancethat a local area network (LAN) can cover and the number of hosts that can be attached to it are limited. To B overcome this limitation, bridges are introduced as devices to connect LANs at the data link layer [ 1 J. The purpose of bridges is to allow hosts attached to different LANs to communicateas ifthey were on the same LAN. Repeaters, devices that act at the physical layer, allow traffic to cross LAN segments, and all traffic appears on all LAN segments. Bridges, on the other hand, should be more intelligent and should limit traffic to the section of the network on which it is relevant. To accomplish this, bridges must make a routing decision upon each received frame as where to send it to reach its destination( s). This article compares two routing algorithms proposed for a bridged LAN environment. One is based on creating a spanning tree topology, as introduced in [2]; the other takes a source routing approach, as described in [ 31. We identlfy the features of the running environment and the functional requirements of bridge routing, followed by a discussion of the two different approaches. 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 features but its topology may be more dynamic than in a single-IAN case due to possible bridge or LAN failures and hosts being moved around. We consider that bridge routing algorithms should meet the following requirements: A bridged LAN environment should resemble a single-IAN environment as closely as possible. In other words, the extension should be transparent to hosts. The transparency requirement extends to perfor- mance requirements, such as low transmission delays, low undetected data corruption, and keeping frames in order. Bridge routing algorithms should be able to adapt quickly to environmental changes. Features of the Two Approaches In this section, first, we will describe each of the two approaches briefly, followed by a discussion of the requirements, cost, and functional limitations of each. Feutures of the Spanning Tree Algorithm In this approach, bridges route each frame according to the destination address. To keep frames from looping, a January 1988-Vo1.2, No.1 IEEE Network loop-free topology is required. Therefore, bridges must first prune an arbitrarily connected physical topology into a logical spanning tree (ST), such as through running a bridge-to-bridge protocol described in [4]. A bridge listens promiscuously to every frame trans- mitted on all of its connected LANs. It examines the source address to learn the direction of the source host and keeps a cache table ofhost-ID/direction pairs (up to a maximum number of pairs as bounded by the hardware). To route a frame, the bridge examines the destination address and looks up its cache. If the destination is found in the cache, the frame is forwarded in the corresponding direction or discarded when the direction is the same as the frame from which it was received; otherwise, it is broadcast in all directions except the,one from which it came. In this approach, bridges are invisible to host stations. No additional host protocol is required beyond that already specified by IEEE 802 for frame data communications across a single LAN. However, the approach does not fully utilize the available resources (because certain channels are disabled to eliminate loops in the topology) and it does not route frames on optimal paths. The ST scheme places the following requirements on bridges: 1 .) Each bridge needs a unique ID that will be used in 2.) Each bridge may need to maintain a large cache of 3.) Bridges must run at a high speed to keep up with The total cost of the ST approach is comprised of three the spanning tree creation protocol. hosts. the LAN’s data transmission rate. parts: 1 .) The cost of running the spanning tree protocol- In the absence of topology changes, one control message is transmitted on each LAN with a settable period; during the period immediately 44 0890-8044/88/0001-0$01.00@ 1988 IEEE Authorized licensed use limited to: MIT Libraries. Downloaded on February 11, 2010 at 15:22 from IEEE Xplore. Restrictions apply.