Spanning tree and vlans, Figure 22: vlan fragmentation, Class 1 laser product – Allied Telesis AT-S63 User Manual
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Chapter 21: Spanning Tree and Rapid Spanning Tree Protocols
230
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 22. 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 22. 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 23, “Port-
based and Tagged VLANs” on page 253.)
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Port
Blocked Data Link
FAULT
RPS
MASTER
POWER
CLASS 1
LASER PRODUCT
STATUS
TERMINAL
PORT
1
3
5
7
9
11
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
9
11
13
15
17
19
21
23R
2
4
6
8
10
12
14
16
18
20
22
24R
23
24
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
1
3
5
7
9
11
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
9
11
13
15
17
19
21
23R
2
4
6
8
10
12
14
16
18
20
22
24R
23
24
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