Understanding the network.pdf - Back to Home

forwarded by bridge 1), it assumes that its (bridge 2) SAT is wrong,
and updates its SAT to reflect that the location of host dog is on
network B. To make matters worse, because it still has no SAT entry
for the host cat, bridge 2 then forwards the packet to network A again.
While all this is going on with bridge 2, the original packet that bridge
2 copied and forwarded has now reached bridge 1. The boomerang of
the original packet causes a repeat performance of the same incorrect
process thathas just occurred on bridge 2.
Now both packets are back on network A, where the same mistake is
made all over again. With each loop, the packets proliferate, and the
packet looks the same each time it is retransmitted.
To prevent loops from occurring, the spanning tree algorithm is used.
According to its creator Radia Perlman (from her book
Interconnections: Bridges and Routers), its purpose "is to have
bridges dynamically discover a subset of the topology that is loop-free
(a tree) and yet has enough connectivity so that where physically
possible, there is a path between every pair of LANs (the tree is
spanning)."
The spanning tree algorithm became IEEE standard 802.1d in 1990. It
was specifically designed to address the looping and packet
replication issues that result from multiple transparent bridge paths.
The spanning tree is created when all the bridges on the LAN elect a
root bridge. After the root bridge is selected, all the other bridges
connected to the LAN calculate the shortest possible distance to the
root bridge. If a bridge has multiple paths, the other paths are placed
in a "blocking state" until the primary path fails; then, the next
shortest path will become active. A spanning tree bridge map is
illustrated in Figure 6.5.