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 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