Allied Telesis AT-GS950/24 - Manual (Page 120)

Chapter 8: STP and RSTP
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If you decide to activate spanning tree on the switch, Allied Telesis
recommends RSTP instead of STP even when all of other switches in the
network are running STP. The AT-GS950/24 switch can combine RSTP
with the STP of the other switches. The switches monitors the traffic on
each port for BPDU packets. Ports that receive RSTP BPDU packets
operate in RSTP mode while ports receiving STP BPDU packets operate
in STP mode.
Spanning Tree
and VLANs
The spanning tree implementation in the AT-S109 Version 1.1.0
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 9, “Multiple Spanning Tree Protocol” on page
133.
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.