Allied Telesis AT-S105 - Manual (Page 153)

AT-S105 Management Software Web Browser User’s Guide
153
Spanning Tree and VLANs
The spanning tree implementation in the AT-S105 Management Software
is a single-instance spanning tree. The AT-FS750/24POE and AT-FS750/
48 switches both support just one spanning tree. You cannot define
multiple spanning trees on either switch.
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 can pose a problem in networks containing multiple
VLANs that span two bridges and are connected with untagged ports. In
this situation, spanning tree blocks a data link because it detects a
suspected data loop. This can cause fragmentation of your VLANs.
This issue is illustrated in Figure 42. VLANs 1 – 3 span two switches. One
link consisting of untagged ports connect each VLAN. If STP or RSTP is
activated on the switches, two of the links are disabled. As a direct result,
two VLANs are disconnected between the bridges. In this example, the
ports (on the non-root switch) that link the two parts of the VLANs 2 - 3 are
changed to the blocking state, which disrupts these VLAN connections.
Figure 47. STP and VLAN Fragmentation with Untagged Ports
VLAN1 VLAN3 VLAN2
Blocked Data Links
Ports Blocked by STP
U U U
U U U
VLAN1 VLAN3 VLAN2