This section describes a basic, 802.1D STP (Spanning Tree Protocol) configuration. Multiple STPDs illustrates a network that uses VLAN (Virtual LAN) tagging for trunk connections.
The following four VLANs have been defined:
Two STPD (Spanning Tree Domain)s are defined:
The carrier and protected VLANs are also defined:
When the switches in this configuration boot-up, STP configures each STPD such that the topology contains no active loops. STP could configure the topology in a number of ways to make it loop-free.
In Multiple STPDs, the connection between switch A and switch B is put into blocking state, and the connection between switch Y and switch Z is put into blocking state. After STP converges, all the VLANs can communicate, and all bridging loops are prevented.
The protected VLAN Marketing, which has been assigned to both STPD1 and STPD2, communicates using all five switches. The topology has no loops, because STP has already blocked the port connection between switch A and switch B and between switch Y and switch Z.
Within a single STPD, you must be extra careful when configuring your VLANs. Incorrect Tag-Based STPD Configuration 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.
The tag-based network in the following figure has the following configuration:
STP can 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.
Note
If an STPD contains multiple VLANs, all VLANs should be configured on all ports in that domain, except for ports that connect to hosts (edge ports).
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