Allied Telesis AT-S111 manual Spanning Tree and VLANs, Ports blocked by STP Blocked Data Links

Chapter 4: STP and RSTP

Spanning Tree and VLANs

in STP mode.

The spanning tree implementation in the AT-S111 Management software can be a single-instance spanning tree as described in this chapter. If you choose to define multiple spanning trees on this switch, go to Chapter 5, “Multiple Spanning Tree Protocol” on page 81.

The single spanning tree encompasses all ports on the switch. If the ports are divided into different VLANs, the spanning tree crosses the VLAN boundaries. This can pose a problem in networks containing multiple VLANs that span two bridges and are connected with untagged ports. In this situation, spanning tree blocks a data link because it detects a suspected data loop. This can cause fragmentation of your VLANs.

This issue is illustrated in Figure 42. VLANs 1 – 3 span two switches. One link consisting of untagged ports connect each VLAN. If STP or RSTP is activated on the switches, two of the links are disabled. As a direct result, two VLANs are disconnected between the bridges. In this example, the ports (on the non-root switch) that link the two parts of the VLANs 2 - 3 are changed to the blocking state, which disrupts these VLAN connections.

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Ports blocked by STP

Blocked Data Links

Figure 19. STP and VLAN Fragmentation with Untagged Ports

You can avoid this problem by connecting the switches using tagged instead of untagged ports when you plan to have STP or RSTP enabled on your network. If each port connecting the two bridges is a tagged member of all three VLANs, then traffic for each of the VLANs can still flow through one the data links if the other two are blocked by Spanning Tree. The second and third data links act as redundant links in case the primary, unblocked data link becomes disabled. See Figure 20 for an example of this solution.

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