Basic STP Configuration

This section describes a basic, 802.1D STP configuration. Multiple STPDs illustrates a network that uses VLAN tagging for trunk connections.

The following four VLANs have been defined:

• Sales is defined on switch A, switch B, and switch M.
• Personnel is defined on switch A, switch B, and switch M.
• Manufacturing is defined on switch Y, switch Z, and switch M.
• Engineering is defined on switch Y, switch Z, and switch M.
• Marketing is defined on all switches (switch A, switch B, switch Y, switch Z, and switch M).

Two STPDs are defined:

• STPD1 contains VLANs Sales and Personnel.
• STPD2 contains VLANs Manufacturing and Engineering.

The carrier and protected VLANs are also defined:

• Sales is the carrier VLAN on STPD1.
• Personnel is a protected VLAN on STPD1.
• Manufacturing is a protected VLAN on STPD2.
• Engineering is the carrier VLAN on STPD2.
• Marketing is a member of both STPD1 and STPD2 and is a protected VLAN.

Multiple STPDs

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.

Incorrect Tag-Based STPD Configuration

The tag-based network in the following figure 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 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.