Moxa Technologies EDS-508 user manual STP Example, Using STP on a Network with Multiple VLANs

EDS-508 Series User’s Manual Featured Functions

STP Example

The figure below shows a LAN that has STP enabled. The LAN has three segments, and each segment is connected using two possible links.

#Bridge A has the lowest Bridge Identifier in the network, and has therefore been selected as the Root Bridge.

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

#Ports 1 of Bridges B, C, X, and Y have been defined as Root Ports because they are the nearest to the Root Bridge and therefore have the most efficient path.

#Bridges B and X offer the same Root Path Cost for LAN segment 2. However, Bridge B has been selected as the Designated Bridge for the segment because it has a lower Bridge Identifier. Port 2 on Bridge B is therefore selected as the Designated Bridge Port for LAN Segment 2.

#Bridge C has been selected as the Designated Bridge for LAN segment 3, because it offers the lowest Root Path Cost for LAN Segment 3:

$the route through Bridges C and B costs 200 (C to B=100, B to A=100)

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

Port 2 on Bridge C is therefore selected as the Designated Bridge Port for LAN Segment 3.

Using STP on a Network with Multiple VLANs

IEEE Std 802.1D, 1998 Edition, does not take into account VLANs when it calculates STP information—the calculations are only performed on the basis of physical connections. For this reason, some network configurations can result in VLANs being subdivided into a number of isolated sections by the STP system. Therefore, you must ensure that any VLAN configuration on your network takes into account the expected STP topology and alternative topologies that may

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