Chapter 9: Rapid Spanning Tree Protocol (RSTP)
Rapid Spanning Tree and VLANs
compatible mode, the ports can receive either RSTP or STP BPDU packets.
The spanning tree implementation in the
The single spanning tree encompasses all ports on the switch. If the ports are divided into different VLANs, the spanning tree crosses the VLAN boundaries. This point can pose a problem in networks containing multiple VLANs that span different switches and are connected with untagged ports. In this situation, STP blocks a data link because it detects a data loop. This can cause fragmentation of your VLANs.
This issue is illustrated in Figure 29. Two VLANs, Sales and Production, span two switches. Two links consisting of untagged ports connect the separate parts of each VLAN. If RSTP is activated on the switches, one of the links is disabled. In the example, the port on the top switch that links the two parts of the Production VLAN is changed to the block state. This leaves the two parts of the Production VLAN unable to communicate with each other.
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Figure 29. VLAN Fragmentation
You can avoid this problem by not activating rapid spanning tree or by connecting VLANs using tagged port members instead of untagged ports. (For information on tagged and untagged ports, refer to Chapter 7, “Virtual LANs” on page 71.)
114 | Section I: Using the Menus Interface |
