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The first step in configuring or troubleshooting Spanning Tree is to ensure your VLANs and trunks are configured properly. Although we naturally think of Spanning Tree as an answer to loops caused by redundant physical connections, bridging loops actually only occur when a layer 2 loop exists in a VLAN. It's possible—indeed, preferable—to configure your trunks in such a way that they don't form a loop to begin with. We'll cover design considerations around VLANs and trunks in Chapter 3. But for Spanning Tree, recognize that the first step is to figure out what VLANs you should have on each switch and what trunks those VLANs should traverse.

Most networks have multiple VLANs stretching across 802.1Q trunks. Ideally, you'll selectively prune VLANs from trunks where you don't need those VLANs. Because any given trunk may carry some VLANs and not others, it's possible for there to be layer 2 loops in one VLAN but not in another. Consider the two VLANs in Figure 2.3. All trunks carry VLAN 3, creating a loop. But only the trunks between SW1 and SW2, SW3, and SW4 carry VLAN 2.

Figure 2.3 Multiple VLANs over 802.1Q trunks

To eliminate the bridging loops in VLAN 3, Spanning Tree must block VLAN 3 traffic from traversing two links. (As a fun exercise, see if you can figure out which two.) VLAN 2 has no loops, so there's no need to block anything. Although we typically say that STP “blocks a port,” this isn't quite accurate. If Spanning Tree were to truly block a port, it would block traffic for all VLANs, not just the one with the loop. This isn't what happens. What it does to break the loop is drop Ethernet frames ingressing a particular port on a particular VLAN. Hence, in order for Spanning Tree to be useful in a multi-VLAN environment, it must treat each VLAN independently.

Per-VLAN Spanning Tree+ (PVST+) is Cisco's implementation of the original Spanning Tree specification, IEEE 802.1D. PVST+ creates a separate Spanning Tree instance for each VLAN. You can configure each Spanning Tree instance differently so that a port blocked for one VLAN will be forwarding for another. This way, you can simultaneously use all available physical links. PVST+ is also sometimes called Shared Spanning Tree Protocol (SSTP).

RPVST+ performs the same basic function as PVST+ but has a faster convergence time—the speed at which each switch settles on which ports to block. As of IOS 15.2(4)E, RPVST+ is the default mode.

MST (802.1s) is similar to RPVST+, except instead of having one Spanning Tree instance per VLAN, MST lets you choose the specific VLANs that belong to each Spanning Tree instance. It's useful for when you have a large number of VLANs and want to minimize the number of Spanning Tree instances to conserve switch resources. In order for a pair of switches to run a PVST+, RPVST+, or MST instance for a VLAN, you need a confluence of several things:

All switches in the topology must have the VLAN configured. It sounds obvious, but a Spanning Tree instance can't participate on a VLAN that doesn't exist. The VLAN must be configured locally on the switch before Spanning Tree can become active for that VLAN.

All switches must have trunks between them. A trunk is always between only two switches. For both switches to establish a trunk, two things must occur:

Both switches must be configured to form a trunk.

Both must agree on the trunk encapsulation type to use 802.1Q or Cisco Inter-Switch Link (ISL).

The VLANs must be allowed on the trunk. Simply establishing a trunk isn't sufficient to carry traffic for a VLAN. Both switches must have the VLAN allowed on the trunk. By default, all VLANs are allowed on a trunk, but it's possible to remove or prune a VLAN from a trunk. We'll look at an example of this in a moment.