User Documentation

User Manual Managed Switches

 Bridge A has been selected as the Root Bridge, since it was determined to have the lowest

Bridge Identifier on the network.

 Since 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.

 Ports 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.

 Bridges 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.

 Bridge C is the Designated Bridge for LAN segment 3, because it has 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)

 The 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.

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.