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.

The STA uses a conclusion from graph theory as a basis for constructing a loop-free subset of the network's topology. Graph theory states the following: "For any connected graph consisting of nodes and edges connecting pairs of nodes, there is a spanning tree of edges that maintains the connectivity of the graph but contains no loops."

Figure 20-3 illustrates how the STA eliminates loops. The STA calls for each bridge to be assigned a unique identifier. Typically, this identifier is one of the bridge's Media Access Control (MAC) addresses plus a priority. Each port in every bridge is also assigned a unique (within that bridge) identifier (typically, its own MAC address). Finally, each bridge port is associated with a path cost. The path cost represents the cost of transmitting a frame onto a LAN through that port. In Figure 20-3, path costs are noted on the lines emanating from each bridge. Path costs are usually default values, but they can be assigned manually by network administrators.

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