Spanning Tree Protocol (STP) and VLAN Interaction

You may encounter an issue when implementing a VLAN on a network which supports Spanning Tree Protocol (STP); STP was built to interact with only one VLAN.

When STP was developed, the concept of a VLAN had not even entered anyone’s thoughts. When VLANs started to become popular, the issue with mixing these two technologies on the same network became apparent; STP only expects to support one network.

Examine the following figure for a moment. If you had to live with the original version of STP, the port that will be blocked is identified on switch 11:55. In this implementation, VLAN1 is configured to run on all network segments, but VLAN2 is implemented on only two of the three network segments. However, now spanning tree has blocked access to the two portions of VLAN2.

The upshot is that computers on these segments, if they are part of VLAN2, will not be able to communicate. To resolve this problem, Per VLAN Spanning Tree (PVST) implements an instance of STP for each VLAN. With PVST, the STP topology for VLAN1 will remain the same; but because VLAN2 is implanted on two segments, it will not form a loop because there is a break in the network between switches 11:11 and 11:22.

In the case of the STP topology for VLAN2, there would be no Blocking Ports.

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When providing VLAN support for STP you encounter two protocols on your Cisco switches:

  • MSTP: An open standards version of STP that allows you to initiate multiple copies of STP on your switch and manually configure separate zones or implementations. MSTP uses the same basic frame structure as RSTP but after the RSTP data in the frame concludes, MSTP adds an additional section to support VLANs on your network. This structure makes it backwards compatible with RSTP, which may be running on your network.

  • PVST: Per VLAN Spanning Tree is more popular than MSTP, especially if a very structured VLAN solution is implemented on your network. Per VLAN creates an instance of STP for each VLAN running on your network, up to a limit of 64 VLANs. If you need to implement more than 64 VLANs on your network, only 64 VLANs can have an instance of STP operating.

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