Spanning Tree And Vlans - Allied Telesis AT-8100L/8 User Manual

Chapter 53: STP, RSTP and MSTP Protocols

Spanning Tree and VLANs

STP and RSTP support a single-instance spanning tree that
encompasses all the ports on the switch. If the ports are divided into
different VLANs, the spanning tree protocol crosses the VLAN boundaries.
This point can pose a problem in networks that contain multiple VLANs
that span different switches and that are connected with untagged ports.
In this situation, STP and RSTP might block a data link if they detect a
data loop, causing fragmentation of your VLANs.
This issue is illustrated in Figure 152. Two VLANs, Sales and Production,
span two switches. Two links consisting of untagged ports connect the
separate parts of each VLAN. If STP or RSTP is activated on the switches,
one of the links is disabled because the links form a loop. In the example,
the port on the top switch that links the two parts of the Production VLAN
is changed to the block state. This leaves the two parts of the Production
VLAN unable to communicate with each other.
Figure 152. VLAN Fragmentation
You can avoid this problem by not activating spanning tree or by
connecting VLANs using tagged instead of untagged ports. (For
information about tagged and untagged ports, refer to Chapter 60, "Port-
based and Tagged VLANs" on page 895.)
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