Specifications

Chapter 6. IBM Virtual Fabric 10Gb Switch Module implementation 269
6.4.3 Spanning Tree Protocol
The STP used for the reference architecture is Per-VLAN Rapid Spanning Tree (PVRST).
PVRST mode is based on RSTP, which provides rapid Spanning Tree convergence, but
allows for multiple Spanning Tree Groups (STGs), with STGs on a per-VLAN basis. PVRST
mode is compatible with Cisco R-PVST/R-PVST+ mode.
To simplify the switch configuration, VLAN Automatic STG Assignment (VASA) can be used
in SPT/PVST+ or PVRST modes. When VASA is enabled, it is no longer necessary to
manually assign an STG for each new VLAN. Instead, each newly configured VLAN is
automatically assigned its own STG. If an empty STG is not available, the VLAN is
automatically assigned to the default VLAN. When a VLAN is deleted, if there is no other
VLAN associated with the assigned STG, the STG is returned to the available pool.
Up to 128 STGs can be configured on the switch (STG 128 is reserved for management).
VASA is disabled by default, but can be enabled or disabled by running spanning-tree
stg-auto as follows:
ACC-3(config)#spanning-tree stg-auto
Warning: all VLANs will be assinged to a STG automatically.
ACC-3(config)#
VASA applies only to STP/PVST+ and PVRST modes and is ignored in RSTP and MSTP
modes. When VASA is enabled, manual STG assignment is still available. The administrator
may assign a specific STG to a VLAN by using regular commands:
# spanning-tree stp <STG> vlan <vlan ID>
When changing to STP/PVST+ or PVSRT mode (either from RSTP or MSTP modes, or when
STP is disabled), all existing VLANs are assigned to a unique STG.
For simplicity and the consistency of the numbering conventions of the reference
architecture, the VLANs IDs are assigned numbers lower than 127 so that the VASA can also
assign matching STG numbers for each VLAN (see Example 6-15 on page 261).
The STG configuration is shown in Example 6-26.
Example 6-26 VLANs and STGs configuration
vlan 30
enable
name "SRV-3"
member 1:14,1:17-1:18
member 2:14,2:17-2:18
spanning-tree stp 30 vlan 30
PVRST is configured in our reference architecture for VLAN30. The Spanning Tree
configuration is consistent with the RackSwitch implementation in Chapter 5, “IBM System
Networking RackSwitch implementation” on page 155.
The per-VLAN roles of the switches are:
AGG-1 (G8264) VLAN 30 PVRST primary root (priority 0)
AGG-2 (G8264) VLAN 30 PVRST secondary root (priority 4096)
ACC-3 (VFSM stack) VLAN 30 default PVRST priority 61440