The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

5.1. Diary of a Loopy LAN

Consider the situation depicted in Figure 5-1. In this scenario, we have two LANs interconnected by two bridges that are effectively in parallel.

Figure 5.1. Two LANs, two bridges, one problem

Let's make a few assumptions:

• Both bridges are "vanilla" (i.e., they don't have any special features or characteristics designed to accommodate this specific situation).

• The address tables within both bridges, at least initially, properly reflect the relative locations of the stations shown. (We will see in a moment that this condition is difficult, if not impossible, to achieve.)

• Both bridges operate according to the principles discussed in Chapter 2, "Transparent Bridges," with respect to frame filtering, forwarding, and address learning.

Given these assumptions, what will be the resulting behavior?

If station 46 sends a frame to station 17, each bridge will receive the frame on (its respective) port 1. Each bridge will perform a Destination Address lookup and determine that the frame should be queued for transmission on port 2. Ultimately (when each bridge's queue empties sufficiently) the frame will be transmitted on port 2 and received by station 17. Thus, station 17 will receive two copies of every unicast frame destined for it that originates on LAN I. In fact, every station on LAN II will receive two copies of every appropriately addressed unicast frame originating ...