Moxa Technologies EDS-518A Series STP Example, Using STP on a Network with Multiple VLANs

EDS-518A Series User’s Manual Featured Functions

STP Example

The LAN shown in the following figure 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				Port 1					Port 1
	(Designated				(Root Port)					(Root Port)
	Bridge Port)
				Cost	=100				Cost	=100
Bridge A				Bridge B					Bridge X
	Port 2				Port 2					Port 2
	(Root Bridge)				(Designated					(Blocked Port)
					Bridge Port)
						LAN Segment 2
				Port 1					Port 1
				(Root Port)					(Root Port)
		Cost	=100				Cost	=200
		Bridge C					Bridge Y
				Port 2					Port 2
				(Designated					(Blocked Port)
				Bridge 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 sine 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 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.

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