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Chapter 4: STP and RSTP
Spanning Tree
and VLANs
82
The spanning tree implementation in the 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 91.
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 26 on page 82. 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.
Figure 26. 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 of 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 27 on page 83
for an example of this solution.
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Ports blocked by STP
Blocked Data Links
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