Specifications

Chapter 5. IBM System Networking RackSwitch implementation 179
5.2.3 Spanning Tree Protocol
The Spanning Tree Protocol used for the reference architecture is Per-VLAN Rapid Spanning
Tree (PVRST). PVRST mode is based on RSTP, which provides rapid Spanning Tree
convergence, and 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 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, and 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:
AGG-1(config)#spanning-tree stg-auto
Warning: all VLANs will be assinged to a STG automatically.
AGG-1(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 running regular commands, such as the
following one:
# 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 consistency of numbering conventions in our example, the VLAN IDs are
assigned numbers lower than 127 (as shown in Example 5-10 on page 164) so the VASA can
also assign matching STG numbers for each VLAN. The STG configuration is shown
in Example 5-30.
Example 5-30 VLANs and STGs configuration
vlan 100
enable
name "AGG1-AGG2"
member 1,5
vlan 101
enable
name "AGG1-ACC1"
member 17-18
vlan 103
enable
name "AGG1-ACC2"
member 19-20
spanning-tree stp 100 vlan 100
spanning-tree stp 101 vlan 101
spanning-tree stp 103 vlan 103