User`s guide

STP Configurations
6-6
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.