Bridge Model

LAN Environment

Bridge Model

In a real LAN topology, bridges operate in the data link layer of the OSI Reference model [Tanenbaum 1996]; therefore, they ignore any network layer information in the data packets. A bridge buffers each packet in its memory and then decides if it needs to forward the packet onto another LAN. Different types of bridges use different algorithms for determining when and where to forward packets. All of these algorithms keep an internal cache containing packet forwarding information. The algorithms' details can be found in [Tanenbaum 1996] and [Perlman 1992]. The Bridge model in netWorks uses a spanning tree algorithm.

Sample Bridge model controls are shown in Figure 4.12.

Figure 4.12: Sample Bridge Model Controls

Since a bridge buffers its packets before it forwards or drops them, the amount of memory a bridge contains can have a significant impact on its performance. If its buffers are full, it could end up dropping or missing packets it should be forwarding. The current simulation routines, however, ignore the Memory property and assume that the device has infinite memory.

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