Moxa PM-7200-4M12 [31/36] Configuring stp rstp

Communication Redundancy STP/RSTP/MSTP

4-6

Using STP on a Network with Multiple VLANs

IEEE Std 802.1D, 1998 Edition, does not take into account VLANs when calculating STP information—the

calculations only depend on the physical connections. Consequently, some network configurations will result in

VLANs being subdivided into a number of isolated sections by the STP system. You must ensure that every

VLAN configuration on your network takes into account the expected STP topology and alternative topologies

that may result from link failures.

The following figure shows an example of a network that contains VLANs 1 and 2. The VLANs are connected

using the 802.1Q-tagged link between switch B and Switch C. By default, this link has a port cost of 100 and

is automatically blocked because the other switch-to-switch connections have a port cost of 36 (18+18). This

means that both VLANs are now subdivided—VLAN 1 on switches A and B cannot communicate with VLAN 1 on

switch C, and VLAN 2 on switches A and C cannot communicate with VLAN 2 on switch B.

To avoid subdividing VLANs, all inter-switch connections should be made members of all available 802.1Q

VLANs. This will ensure connectivity at all times. For example, the connections between switches A and B, and

between switches A and C, should be 802.1Q tagged and carrying VLANs 1 and 2 to ensure connectivity.

Configuring STP/RSTP

The following figures indicate which Spanning Tree Protocol parameters can be configured. A more detailed

explanation of each parameter follows.