7-4 S
UMMIT
S
WITCH
I
NSTALLATION
AND
U
SER
G
UIDE
S
PANNING
T
REE
P
ROTOCOL
(STP)
Within a single STPD, you must be extra careful when configuring your VLANs.
Figure 7-2 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 7-2:
Tag-based STP configuration
The tag-based network in Figure 7-2 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.
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
Summit.bk : 7STP.FM Page 4 Thursday, June 18, 1998 9:27 AM
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