Moxa IKS-6728A-4GTXSFP-24-24-T [25/37] Using stp on a network with multiple vlans

Managed Ethernet Switch Redundancy Protocol (UI 2.0) STP/RSTP/MSTP

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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

Use the scrolling bar at the top of the Redundancy Protocol page to select among Turbo Ring, Turbo Ring V2,

Turbo Chain, RSTP, or MSTP. Note that configuration pages for these five protocols are different.

Protocol

Setting Description Factory Default

Turbo Ring Select this item to change to the Turbo Ring configuration page.

None

Turbo Ring V2

Select this item to change to the Turbo Ring V2 configuration

page.

Turbo Chain Select this item to change to the Turbo Chain configuration page.

RSTP (IEEE

802.1D-2004)

Select this item to change to the RSTP configuration page.

MSTP (IEEE 802.1s) Select this item to change to the MSTP configuration page.