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There's nothing inherently problematic about having physical loops. The physical loops become problematic only when they create layer 2 loops. In Figure 2.1, all links are in VLAN 1, creating a loop between each pair of switches as well as among them. We could easily eliminate these layer 2 loops in two ways:

 By removing the physical loops

 By doing away with a stretched VLAN and turning each link into a routed (layer 3) link, where each link represents a separate broadcast domain

Figure 2.1 Physical looped topology

The latter option would eliminate any chance of a broadcast storm and, from a scalability and stability perspective, would be ideal. We'll cover routed topologies in Chapter 3, “Enterprise Network Design.” Although a routed topology is ideal, implementing it requires planning and effort that the organization may not be willing or able to spend. Many therefore extend VLANs across several switches connected together, often in a loop for redundancy.

The goal of STP is to impose a loop-free logical topology by strategically dropping certain Ethernet frames on specific interfaces (what STP calls ports) to prevent them from endlessly looping through the network. You should already have at least a passing familiarity with Spanning Tree. Before continuing, take a moment to test your understanding by trying to calculate the root bridge and root ports for the topology in Figure 2.1.

To refresh your memory, the STP process is as follows:

1 All switches elect a root bridge. The root bridge doesn't block any traffic but forwards frames normally.

2 Each non-root switch determines its root port. The root port is the port that provides the lowest-cost path back to the root bridge. Any other ports leading back to the root block frames to prevent a loop from forming between the root and the non-root bridge.

3 Each connected pair of non-root switches elects a designated bridge for that link or segment. The designated bridge forwards frames over that link normally, whereas the other bridge doesn't forward any frames. The result is a logical loop-free topology, as shown in Figure 2.2.

Figure 2.2 Converged STP topology with SW1 as the root