User guide

6-4 B
LACK
D
IAMOND
U
SER
G
UIDE
S
PANNING
T
REE
P
ROTOCOL
(STP)
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.
STP may block traffic between Switch 1 and Switch 3 by disabling the trunk ports for
that connection on each switch.
Marketing & Sales Marketing, Sales & Engineering
Switch 1 Switch 3
Switch 2
Sales & Engineering
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