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Chapter 25: Spanning Tree and Rapid Spanning Tree Protocols
280 Section V: Spanning Tree Protocols
Spanning Tree and VLANs
The STP and RSTP implementations in the AT-S63 Management
Software support a single-instance spanning tree that encompasses all
the 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 that contain multiple VLANs that span different
switches and that are connected with untagged ports. In this situation,
STP and RSTP might block a data link if they detect a data loop, causing
fragmentation of your VLANs.
This issue is illustrated in Figure 36. 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 because the links form a loop. 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 36. 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 27, “Port-
based and Tagged VLANs” on page 311.)
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 9 11 13 15 17 19 2123R
2 4 6 8 1012 1416 1820 2224R
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
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 9 11 13 15 17 19 2123R
2 4 6 8 1012 1416 1820 2224R
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