User`s manual

IKS-6726 Featured Functions

3-33

• 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 sine 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 Switch units A and B cannot communicate with VLAN

1 on Switch C, and VLAN 2 on Switch units A and C cannot communicate with VLAN 2 on Switch B.