Allied Telesis AT-S82 - Manual (Page 65)

AT-S82 Management Software User’s Guide
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If you decide to activate spanning tree on the switch, there is no reason
not to activate RSTP on an AT-GS950/8 WebSmart switch even when all
other switches are running STP. The switch can combine its RSTP with
the STP of the other switches. The switch monitors the traffic on each port
for BPDU packets. Ports that receive RSTP BPDU packets operates in
RSTP mode while ports receiving STP BPDU packets operate in STP
mode.
Spanning Tree
and VLANs
The spanning tree implementation in the AT-S82 management software is
a single-instance spanning tree. The switch supports just one spanning
tree. You cannot define multiple spanning trees.
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 point can pose a problem in networks containing multiple
VLANs that span different switches and are connected with untagged
ports. In this situation, STP blocks a data link because it detects a data
loop. This can cause fragmentation of your VLANs.
This issue is illustrated in Figure 27. Two VLANs, Sales and Production,
span two AT-GS950/8 WebSmart 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. 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 27. VLAN Fragmentation
You can avoid this problem by not activating spanning tree or by
connecting VLANs using tagged instead of untagged ports. (For
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Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Port
Blocked Data Link