Allied Telesis AT-GS950/8 manual Spanning Tree and VLANs

Page 69

Spanning Tree and VLANs

AT-S82 Management Software User’s Guide

If you decide to activate spanning tree on the switch, there is no reason not to activate RSTP on an AT-GS950/8 WebSmart switch even when all other switches are running STP. The switch can combine its RSTP with the STP of the other switches. The switch monitors the traffic on each port for BPDU packets. Ports that receive RSTP BPDU packets operates in RSTP mode while ports receiving STP BPDU packets operate in STP mode.

The spanning tree implementation in the AT-S82 management software is

asingle-instance spanning tree. The switch supports just one spanning tree. You cannot define multiple spanning trees.

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 28. Two VLANs, Sales and Production, span two AT-GS950/8 WebSmart switches. Two links consisting of untagged ports connect the separate parts of each VLAN. If STP or 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.

Sales

Production

 

VLAN

VLAN

Blocked Port

 

 

912

 

 

Blocked Data Link

Sales

Production

912

 

VLAN

VLAN

 

Figure 28. VLAN Fragmentation

You can avoid this problem by not activating spanning tree or by connecting VLANs using tagged instead of untagged ports. (For

69

Page 68 Page 70
Image 69
Page 68 Page 70
Contents Management Software Page Contents Contents Figures Figures Preface Where to Find Web-based Guides Contacting Allied Telesis Management Software UpdatesPreface Chapter Getting StartedStarting a Management Session Main Saving Changes Save Configuration Click ApplyQuitting a Management Session Getting Started Basic Switch Parameters Basic Switch Parameters AT-S82 Management Software User’s Guide Enabled DisabledDisabling or Enabling Ping Blocking Enabling or Disabling 802.1X Forwarding Control Changing the Administrator’s Password Rebooting the Switch Reset Click Apply Switch is reset Resetting the Switch and Retaining the IP AddressAT-S82 Management Software User’s Guide Download Status Message Downloading New FirmwarePort Configuration Configuring Port Parameters Port is enabled. This is the default Fiber CopperTrunking Trunking Guidelines Trunking Algorithm Trunking OverviewAT-S82 Management Software User’s Guide DAxorSA Configuring the Trunking AlgorithmPort Trunking Configuration Setting up the TrunkTrunking Port Mirroring Port Mirroring Overview Configuring Port Mirroring Port Mirroring VLANs Increased Performance Improved Manageability Security Vlan FeaturesAT-S82 Management Software User’s Guide Vlan Identifier Vlan Port Members Vlan OverviewIncoming Outgoing Tagged Untagged Frames Guidelines for Creating a Vlan Vlan Detail Working with VLANsClick Show All Static Vlan Entries Displaying all VLANsVlan Protected Ports Vlan Example Protected Ports VlanAT-S82 Management Software User’s Guide VLANs Class of Service Mapping Ports to Egress Queues Class of Service OverviewIeee 802.1p Priority Level Port Priority Queue Port Egress Queue Maximum Number Packets Mapping Priorities to Queues Setting Up the Schedule Assigning Priority to Ports Class of Service Spanning Tree Protocol STP Bridge Priority Root Spanning Tree OverviewIncrement Bridge Priority Path Costs and Port CostsPort Speed Port Cost Increment Port Priority Port PriorityHello Time and Bridge Protocol Data Units BPDUs Point-to-Point and Edge Ports Edge Port Spanning Tree and VLANs Spanning Tree Protocol STP Configuring Spanning Tree STP Compatible RstpDisabled Configuring STP Port Settings False YesTrue AutoSpanning Tree Protocol STP
Related manuals
Manual 44 pages 22.06 Kb
Top Page Image Contents