User`s guide

AT-9108, AT-8518, AT-8525, and AT-8550 User’s Guide
6-5
When the switches in this configuration start up, STP configures each
STPD such that there are no active loops in the topology. STP could
configure the topology in a number of ways to make it loop-free.
In Figure 6-1, the connection between Switch A and Switch B is put
into blocking state, and the connection between Switch Y and
Switch Z is put into blocking state. After STP converges, all the VLANs
can communicate, and all bridging loops are prevented.
The VLAN
Marketing
, which has not been assigned to either STPD1 or
STPD2, communicates using all five switches. The topology has no
loops, because STP has already blocked the port connection
between Switch A and Switch B, and between Switch Y and Switch Z.
Within a single STPD, you must be extra careful when configuring
your VLANs. Figure 6-2 illustrates a network that has been incorrectly
set up using a single STPD so that the STP configuration disables the
ability of the switches to forward VLAN traffic.
Figure 6-2
Tag-Based STP Configuration
The tag-based network in Figure 6-2 has the following configuration:
Switch 1 contains VLAN
Marketing
and VLAN
Sales
.
Switch 2 contains VLAN
Engineering
and VLAN
Sales
.
Switch 3 contains VLAN
Marketing
, VLAN
Engineering
, and
VLAN
Sales
.
The tagged trunk connections for three switches form a
triangular loop that is not permitted in an STP topology.
All VLANs in each switch are members of the same STPD.
Marketing & Sales Marketing, Sales & Engineering
Sales & Engineering
Switch 1 Switch 3
Switch 2
Centre
COM
Centre
COM
Centre
COM