Allied Telesis AT-S81 - Manual (Page 144)

Chapter 12: Rapid Spanning Tree Protocol (RSTP)
144 Section II: Menus Interface
set the switch to RSTP mode, all the ports operate in that mode and reject
STP BPDU packets. When you set the switch to operate in STP-
compatible mode, the ports can receive either RSTP or STP BPDU
packets.
Rapid Spanning
Tree and VLANs
The spanning tree implementation in the AT-S81 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 33. Two VLANs, Sales and Production,
span two AT-8000/8POE Fast Ethernet 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 33. 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 9, “Virtual
LANs” on page 95.)
AT-8000/8POE
8 Port POE Fast Ethernet Switch
PoE
9 9R
1 3 5 7
2 4 6 8
TERMINAL POR
T 1 3 5 7
2 4 6 8
9
9R POWER
10/100/1000 Base-T
SFP
UPLINK PORT
RS-232
DET ON ON
100 LINK ACT 10 LINK ACT
PD ON MAX CURRENT PD ERR
10/100 LINK
1000 LINK ACT
ACT CLASS 1 LASER PRODUCT
MAIN PORT ACTIVITY
UPLINK PORT ACTIVITY
820
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Port
Blocked Data Link
AT-8000/8POE
8 Port POE Fast Ethernet Switch
PoE
9 9R
1 3 5 7
2 4 6 8
TERMINAL POR
T 1 3 5 7
2 4 6 8
9
9R POWER
10/100/1000 Base-T
SFP
UPLINK PORT
RS-232
DET ON ON
100 LINK ACT 10 LINK ACT
PD ON MAX CURRENT PD ERR
10/100 LINK
1000 LINK ACT
ACT CLASS 1 LASER PRODUCT
MAIN PORT ACTIVITY
UPLINK PORT ACTIVITY
820