Allied Telesis AT-S79 - Manual (Page 114)

Chapter 9: Rapid Spanning Tree Protocol (RSTP)
114 Section I: Using the Menus Interface
compatible mode, the ports can receive either RSTP or STP BPDU
packets.
Rapid Spanning
Tree and VLANs
The spanning tree implementation in the AT-S79 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 29. Two VLANs, Sales and Production,
span two switches. Two links consisting of untagged ports connect the
separate parts of each VLAN. If 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 29. VLAN Fragmentation
You can avoid this problem by not activating rapid spanning tree or by
connecting VLANs using tagged port members instead of untagged ports.
(For information on tagged and untagged ports, refer to Chapter 7, “Virtual
LANs” on page 71.)
732
1 3 5 7 9 11 13 15 17 19 21 23
POWER 2 4 6 8 10 12 14 16 18 20 22 24 23 24 LINK/ACT
LINK/ACT
SPEED
LINK/ACT
SPEED
AT-GS950/24 24-Port 10/100/1000Mbps + 2 SFP Combo WebSmart Switch
732
1 3 5 7 9 11 13 15 17 19 21 23
POWER 2 4 6 8 10 12 14 16 18 20 22 24 23 24 LINK/ACT
LINK/ACT
SPEED
LINK/ACT
SPEED
AT-GS950/24 24-Port 10/100/1000Mbps + 2 SFP Combo WebSmart Switch
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Port
Blocked Data Link