Understanding the network.pdf - Back to Home

To get a better understanding of this problem, let's look at the
example illustrated in Figure 6.4.
Figure 6.4. A transparent bridge packet loop.
The hosts dog and cat have just come up on network A and network B.
The host dog sends a packet to cat. This packet is copied by both
bridges, 1 and 2. Because the hosts dog and cat are new to the LAN,
neither of them is entered in either of the bridge's SATs. So, both
bridges add the host dog to their SAT tables when they see its packet
hit the port. However, because the host cat has yet to send anything,
its destination is unknown, both bridges blindly send or flood the
packet on to network B.
Because both bridges have to wait until the wire is clear before they
can actually forward the packet, one gets to forward the packet first.
In this case, it is bridge 1, so its packet reaches network B, where it is
immediately copied by bridge 2. It is at this point that the trouble
begins.
Bridge 2 only has an SAT entry for the host dog, and still no entry for
the host cat. The problem starts because bridge 2 believes (correctly,
mind you) that dog is attached to network A. However, sensing that it
has just received a packet from the host dog on network B (just