Allied Telesis AT-S101 - Manual (Page 143)

AT-S101 Management Software User’s Guide
143
Spanning Tree and VLANs
The spanning tree implementation in the AT-S101 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 44. Two VLANs, Sales and Production,
span two AT-GS950/8POE 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 44. VLAN Fragmentation
1369
AT-GS950/8POE 8 Port 10/100/1000Mbps + 2 SFP Combo WebSmart Switch
SFP SFP
CLASS 1 LASER PRODUCT
1 2 3 4 5 6 7R 7 8 8R POWER RESET
1 2 3 4 5 6 7R 8R
7 8
POE
100/1000Base-X
PORT ACTIVITY
ACT
1000 LINK
100 LINK
ACT
ACT
SFP
AT-GS950/8POE 8 Port 10/100/1000Mbps + 2 SFP Combo WebSmart Switch
SFP SFP
CLASS 1 LASER PRODUCT
1 2 3 4 5 6 7R 7 8 8R POWER RESET
1 2 3 4 5 6 7R 8R
7 8
POE
100/1000Base-X
PORT ACTIVITY
ACT
1000 LINK
100 LINK
ACT
ACT
SFP
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Data Link
Blocked Port