Allied Telesis AT-S60 - Manual (Page 238)

Chapter 15: STP and RSTP
Section II: Advanced Features 238
The single spanning tree encompasses all ports on the switch. If the
ports are grouped into different VLANs, the spanning tree crosses the
VLAN boundaries. This can pose a problem where multiple VLANs that
span different switches are connected with untagged ports. What can
occur is that spanning tree blocks a data link because it detects a
physical data loop. This can cause fragmentation of your VLANs.
This is illustrated in Figure 73. Two VLANs, Sales and Production, span
one AT-8400 Series switch and one AT-8024GB switch. Two links
consisting of untagged ports interconnect the separate parts of each
VLAN. If spanning tree is activated on the switches, one of the links
would be disabled because spanning tree, which crosses the VLAN
boundaries, would see the links as forming a physical loop, even though
the VLAN traffic itself does not cross the boundaries.
Figure 73 VLAN Fragmentation
There are several approaches that you can take to resolve this problem.
One is not to activate STP or RSTP on your network. This solution
mandates vigilance on your part not to create network loops when
wiring your network.
Link
Mode
Link
Mode
100
FULL
ACT
MODE
COL FAULT
MASTER
PWR
AT-8024
10Base-T / 100Base-TX Fast Ethernet Switch RS-232 TERMINAL PORT
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
AT-8400
Modular Switch System
2 3 4 5 6 M 7 8 9 10 11 12 1
AT-8401
RS-232 TERMINAL PORT
POWER
FAN A
WAIT
MASTER
FAULT
FAN B
DISCON
SLAVE
RESET
AT-8411
PWR
EJECT
LNK 10 ACT 100
AT-8411
PWR
EJECT
LNK 10 ACT 100
AT-8411
PWR
EJECT
LNK 10 ACT 100
AT-8411
PWR
EJECT
LNK 10 ACT 100
AT-8411
PWR
EJECT
LNK 10 ACT 100