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Take a look at 6-20 more complex topologies to see how STP is actually applied in practice.
Figure 6-20. A Complex Network with all Links shown
Figure 6-20 is a network of highly redundant (i.e., ring)-configured networks connected by 7 switches. The remaining links are Fast Ethernet (overhead is 19), except for the 10Based link (overhead 100) that is upright on the leftmost side.
Assuming Cat-4 wins the root battle, figure 6-21 is its final activity topology.
Figure 6-21. Complex Network with central Root Bridge and Active topology
Figure 6-21 clearly shows the basic constituent body of the spanning Tree protocol: A bridge becomes the center of a network world, and all the bridges to the center have the shortest path ("All roads lead to Rome"). The result is a network activity topology diagram of a spoke-like branch that radiates outward from the root bridge. Note that the root bridge is a data traffic exchange center with four branches and must be able to withstand the increased network load. For example, the flow of Cat-7 and Cat-5 on branch D to any other branch must pass through the root Bridge (Cat-4), i.e.,Cat-4Do not use the slowest bridge. Figure 6-21 also illustrates the importance of the root Bridge center location. Imagine data traffic between the host-a on Cat-7 and the host-b on Cat-6, when two users want to play Doom games, although Cat-6 and Cat-7 are directly connected but data traffic must still pass through four bridges. This may seem very inefficient, but in fact it may be bad! Imagine, for example, 6-22 that Cat-1 won the Battle of the Roots.Figure 6-22. Complex Network with inefficient Root Bridge and Active topology
In this scenario, the network converges to two branches, and all the link data traffic passes through the root bridge. However, notice how the data flow is inappropriate--host-a and host-b between the Doom game data traffic must pass through all 7 bridges. If I didn't tell you not to choose the root bridge randomly, then let me point out how bad the facts are for you. Assume that the Cat-1 is a software bridge made by an earlier Cisco MGS or AGS Router (a 2-tier forwarding capability is approximately equal to 10,000 packets per second), and the remaining six devices are Catalyst 5500 or 6000 switches (2-tier forwarding capability for millions of packets per second). Not to worry, I'm sure MGs will become the root bridge by default. Why am I so sure? Well, what determines who wins the root war? The smallest bid. As you can see earlier, bid is comprised of two subdomains: Bridge priority and MAC address. Because all bridges have a default priority of 32768, the smallest Mac becomes the root bridge by default. The MAC address of the catalyst switch starts with the identification of the oui, such as 00-10-FF and 00-E0-F9 (for example, MAC address 00-10-ff-9f-85-00), all Cisco MGS uses Cisco Legacy OUI for 00-00-0c (for example, MAC address 00-00-0c-58-af-c1). 00-00-0C has a smaller oui--that is only 12 in number. As a result, your MGS will always be smaller than the MAC address of the catalyst switch you buy, so it will always win a root battle in the Cisco Network (as well as other networks on the planet). In other words, if you ignore the location layout of the root bridge, there are probably 1000 factors that can degrade your network throughput. Obviously, manually controlling your root bridge is critical for a well-performing 2-tier network.
The classic "Cisco Lan Switching" chapter sixth (11): Using Spanning Tree in Real-world Networks
