3Com 9100 manual Stpd

92CHAPTER 6: SPANNING TREE PROTOCOL (STP)

Sales, Personnel, Marketing

Switch A

Switch B

STPD 1

Manufacturing, Engineering, Marketing

Switch Y

Switch Z

Switch M STPD 2

Sales, Personnel, Manufacturing, Engineering, Marketing

Figure 16 Multiple Spanning Tree Domains

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When the switches in this configuration start up, STP configures each STPD such that there are no active loops in the topology. STP could configure the topology in a number of ways to make it loop-free.

In Figure 16, the connection between Switch A and Switch B is put into blocking state, and the connection between Switch Y and Switch Z is put into blocking state. After STP converges, all the VLANs can communicate, and all bridging loops are prevented.

The VLAN Marketing, which has not been assigned to either STPD1 or STPD2, communicates using all five switches. The topology has no loops, because STP has already blocked the port connection between Switch A and Switch B, and between Switch Y and Switch Z.

Within a single STPD, you must be extra careful when configuring your VLANs. Figure 17 illustrates a network that has been incorrectly set up using a single STPD so that the STP configuration disables the ability of the switches to forward VLAN traffic.