Radware Alteon - Purpose of Multiple Spanning Trees; Four-Alteon Topology with a Single Spanning Tree

Alteon Application Switch Operating System Application Guide
Spanning Tree Protocol
Document ID: RDWR-ALOS-V2900_AG1302 103
This implementation of RSTP improves upon previous implementations by addressing slow
convergence times.
Note: By default, all newly created VLANs are members of STG1.
For specific information on MSTP and RSTP, see:
• Rapid Spanning Tree Protocol, page 105
• Multiple Spanning Tree Protocol, page 107
Purpose of Multiple Spanning Trees
Figure 7 - Example Multiple Spanning Tree Configuration, page 103 illustrates the purpose of
multiple spanning trees. Two VLANs, VLAN 1 and VLAN 100 exist between Alteon A and Alteon B. If
you have a single STG, the Alteons detect an apparent loop, and one VLAN may become blocked,
affecting connectivity, even though no actual loop exists.
If VLAN 1 and VLAN 100 belong to different STGs, then the two spanning tree instances separate the
topology without forming a loop. Both VLANs can forward packets between the Alteons without
losing connectivity.
Figure 7: Example Multiple Spanning Tree Configuration
Four-Alteon Topology with a Single Spanning Tree
In a four-Alteon topology (see Figure 8 - Four-Alteon Topology with a Single Spanning Tree, page
104), and assuming Alteon A has a higher priority, you can have at least three loops on the network:
• Data flowing from Alteons A to B to C and back to Alteon A.
• Data flowing from Alteons A to C to D and back to Alteon A
• Data flowing from Alteons A to B to C to D and back to Alteon A.
With a single spanning tree environment, you have two links blocked to prevent loops on the
network. It is possible that the blocks may be between Alteons C and D and between Alteons B and
C, depending on the bridge priority, port priority, and port cost. The two blocks would prevent
looping on the network, but the blocked link between Alteons B and C will inadvertently isolate VLAN
3 altogether.
For more information on bridge priority, port priority, and port cost, see the Alteon Application
Switch Operating System Command Reference.