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7-8 CHAPTER 7: SPANNING TREE PROTOCOL (STP)
STP Configurations
to Avoid
Within a single STPD, you must be careful when configuring your
VLANs. The following figures illustrate networks that have been
incorrectly set up so that the STP configuration disables the ability of
the Switches to forward VLAN traffic.
The tag-based network in Figure 7-5 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.
Figure 7-5 Tag-based STP configuration
STP may block traffic between Switch 1 and Switch 3 by disabling the
trunk ports for that connection on each Switch.
Switch 2 has no ports assigned to VLAN marketing. Therefore, if the
trunk for VLAN Marketing on Switches 1 and 3 is blocked, the traffic
for VLAN marketing will not be able to traverse the Switches.
Marketing & Sales Marketing, Sales & Engineering
Sales & Engineering
Switch 1 Switch 3
Switch 2
SW3800.BK Page 8 Tuesday, May 5, 1998 5:20 PM