User guide

7-4 SUMMIT SWITCH INSTALLATION AND USER GUIDE
S PANNING T REE P ROTOCOL (STP)
Within a single STPD, you must be extra careful when conÞguring your VLANs.
Figure 7-2 illustrates a network that has been incorrectly set up using a single STPD so
that the STP conÞguration disables the ability of the Switches to forward VLAN trafÞc.
Figure 7-2: Tag-based STP configuration
The tag-based network in Figure 7-2 has the following conÞguration:
¥ 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 trafÞc 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 trafÞc 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 Page 4 Thursday, September 25, 1997 12:33 PM