Allied Telesis AT-FS970M Series - Manual (Page 763)

AT-FS970M Switch Command Line User’s Guide
763
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 147. 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 147. 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 58, “Port-
based and Tagged VLANs” on page 879.)