Allied Telesis AT-GS950/16PS, Руководство пользователя

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Chapter 4: STP and RSTP
68
Spanning Tree
and VLANs
The spanning tree implementation in the AT-S112 Management software
can be a single-instance spanning tree as described in this chapter. If you
choose to define multiple spanning trees on this switch, go to Chapter 5,
“Multiple Spanning Tree Protocol” on page 79.
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 19. STP and VLAN Fragmentation with Untagged Ports
You can avoid this problem by connecting the switches using tagged
instead of untagged ports when you plan to have STP or RSTP enabled
on your network. If each port connecting the two bridges is a tagged
member of all three VLANs, then traffic for each of the VLANs can still flow
through one the data links if the other two are blocked by Spanning Tree.
The second and third data links act as redundant links in case the primary,
unblocked data link becomes disabled. See Figure 20 for an example of
this solution.
U
U
U
U
U
U
Ports blocked by STP
Blocked Data Links