Moxa Technologies EDS-408A STP Example, Using STP on a Network with Multiple VLANs, 3-31

EDS-408A/405A Series User’s Manual Featured Functions

STP Example

The LAN shown below has three segments, with adjacent segments connected using two possible links. The various STP factors, such as Cost, Root Port, Designated Bridge Port, and Blocked Port are shown in the figure.

LAN Segment 1

Port 1 (Designated Bridge Port)

Bridge A

Port 2

(Root Bridge)

Port 1

(Root Port)

Cost =100

Bridge B

Port 2 (Designated Bridge Port)

Port 1

(Root Port)

Cost =100

Bridge X

Port 2 (Blocked Port)

LAN Segment 2

Port 1

(Root Port)

Cost =100

Bridge C

Port 2 (Designated Bridge Port)

Port 1

(Root Port)

Cost =200

Bridge Y

Port 2 (Blocked Port)

LAN Segment 3

yBridge A has been selected as the Root Bridge, since it was determined to have the lowest Bridge Identifier on the network.

ySince Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment 1. Port 1 on Bridge A is selected as the Designated Bridge Port for LAN Segment 1.

yPorts 1 of Bridges B, C, X, and Y are all Root Ports since they are nearest to the Root Bridge, and therefore have the most efficient path.

yBridges B and X offer the same Root Path Cost for LAN segment 2. However, Bridge B was selected as the Designated Bridge for that segment since it has a lower Bridge Identifier. Port 2 on Bridge B is selected as the Designated Bridge Port for LAN Segment 2.

yBridge C is the Designated Bridge for LAN segment 3, because it has the lowest Root Path Cost for LAN Segment 3:

¾The route through Bridges C and B costs 200 (C to B=100, B to A=100)

¾The route through Bridges Y and B costs 300 (Y to B=200, B to A=100)

yThe Designated Bridge Port for LAN Segment 3 is Port 2 on Bridge C.

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 depend only 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

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