Chapter 20: Spanning Tree and Rapid Spanning Tree Protocols
224 Section V: Spanning Tree Protocols
Spanning Tree and VLANs

The spanning tree implementation in the AT-S63 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 24. Two VLANs, Sales and Production,

span two AT-9400 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 24. VLAN Fragmentation

You can avoid this problem by not activating spanning tree or by

connecting VLANs using tagged instead of untagged ports. (For

information on tagged and untagged ports, refer to Chapter 22, “Port-

based and Tagged VLANs” on page 247.)

Sales

VLAN
Production
VLAN
Production
VLAN

Sales

VLAN

Blocked Port

Blocked Data Link

FAULT
RPS
MASTER
POWER
CLASS 1
LASER PRODUCT
STATUS
TERMINAL
PORT
1357911
2 4 6 8 10 12
13 15 17 19 21 23R
14 16 18 20 22 24R
AT-9424T/SP Gigabit Ethernet Switch
1 3 5 7 911 1315 1719 21 23R
2 4 6 8 10 12 14 16 18 20 2224R
2324
L/A
D/C
D/C
L/A
D/C
L/A
1000 LINK / ACT
HDX /
COL
FDX
10/100 LINK / ACT
PORT ACTIVITY
L/A
1000 LINK / ACT
SFP
SFP
24
SFP
23
FAULT
RPS
MASTER
POWER
CLASS 1
LASER PRODUCT
STATUS
TERMINAL
PORT
1357911
2 4 6 8 10 12
13 15 17 19 21 23R
14 16 18 20 22 24R
AT-9424T/SP Gigabit Ethernet Switch
1 3 5 7 911 1315 1719 21 23R
2 4 6 8 10 12 14 16 18 20 2224R
2324
L/A
D/C
D/C
L/A
D/C
L/A
1000 LINK / ACT
HDX /
COL
FDX
10/100 LINK / ACT
PORT ACTIVITY
L/A
1000 LINK / ACT
SFP
SFP
24
SFP
23