System information

Transparent Bridging Technology Basics
Book Title
20-422
Bridging Loops
Without a bridge-to-bridge protocol, the transparent bridge algorithm fails when there are multiple
paths of bridges and local-area networks (LANs) between any two LANs in the internetwork.
Figure 20-2 illustrates such a bridging loop.
Figure 20-2 Inaccurate Forwarding and Learning in Transparent Bridging Environments
Suppose Host A sends a frame to Host B. Both bridges receive the frame and correctly conclude that
Host A is on Network 2. Unfortunately, after Host B receives two copies of Host A’s frame, both
bridges again receive the frame on their Network 1 interfaces because all hosts receive all messages
on broadcast LANs. In some cases, the bridges will then change their internal tables to indicate that
Host A is on Network 1. If this is the case, when Host B replies to Host A’s frame, both bridges
receive and subsequently drop the replies because their tables indicate that the destination (Host A)
is on the same network segment as the frame’s source.
In addition to basic connectivity problems such as the one just described, the proliferation of
broadcast messages in networks with loops represents a potentially serious network problem.
Referring again to Figure 20-2, assume that Host A’s initial frame is a broadcast. Both bridges will
forward the frames endlessly, using all available network bandwidth and blocking the transmission
of other packets on both segments.
A topology with loops such as that shown in Figure 20-2 can be useful as well as potentially harmful.
A loop implies the existence of multiple paths through the internetwork. A network with multiple
paths from source to destination can increase overall network fault tolerance through improved
topological flexibility.
The Spanning-Tree Algorithm
The spanning-tree algorithm (STA) was developed by Digital, a key Ethernet vendor, to preserve the
benefits of loops while eliminating their problems. Digital’s algorithm was subsequently revised by
the IEEE 802 committee and published in the IEEE 802.1d specification. The Digital algorithm and
the IEEE 802.1d algorithm are not the same, nor are they compatible.
The STA designates a loop-free subset of the network’s topology by placing those bridge ports that,
if active, would create loops into a standby (blocking) condition. Blocking bridge ports can be
activated in the event of primary link failure, providing a new path through the internetwork.
Host A
Host B
Network 2
Network 1
Bridge B
Bridge A