122 RackSwitch G8000: Application Guide
By default, all other STGs (STG 2 through 128) are enabled, though they initially
include no member VLANs. VLANs must be assigned to STGs. By default, this is
done automatically using VLAN Automatic STG Assignment (VASA), though it can
also be done manually (see “Manually Assigning STGs” on page 122.
When VASA is enabled (as by default), each time a new VLAN is configured, the
switch will automatically assign that new VLAN to its own STG. Conversely, when a
VLAN is deleted, if its STG is not associated with any other VLAN, the STG is
returned to the available pool.
The specific STG number to which the VLAN is assigned is based on the VLAN
number itself. For low VLAN numbers (1 through 128), the switch will atempt to
assign the VLAN to its matching STG number. For higher numbered VLANs, the
STG assignment is based on a simple modulus calculation; the attempted STG
number will “wrap around,” starting back at the top of STG list each time the end of
the list is reached. However, if the attempted STG is already in use, the switch will
select the next available STG. If an empty STG is not available when creating a new
VLAN, the VLAN is automatically assigned to default STG 1.
If ports are tagged, each tagged port sends out a special BPDU containing the
tagged information. Also, when a tagged port belongs to more than one STG, the
egress BPDUs are tagged to distinguish the BPDUs of one STG from those of
another STG.
VASA is enabled by default, but can be disabled or re-enabled using the following
commands:
If VASA is disabled, when you create a new VLAN, that VLAN automatically belongs
to default STG 1. To place the VLAN in a different STG, assign it manually.
VASA applies only to PVRST mode and is ignored in RSTP and MSTP modes.
Manually Assigning STGs
The administrator may manually assign VLANs to specific STGs, whether or not
VASA is enabled.
1. If no VLANs exist (other than default VLAN 1), see “Guidelines for Creating
VLANs” on page 123 for information about creating VLANs and assigning ports
to them.
2. Assign the VLAN to an STG using one of the following methods:
– From the global configuration mode:
– Or from within the VLAN configuration mode:
When a VLAN is assigned to a new STG, the VLAN is automatically removed from
its prior STG.
Note: For proper operation with switches that use Cisco PVST+, it is
recommended that you create a separate STG for each VLAN.
RS G8000(config)# [no] spanning-tree stg-auto
RS G8000(config)# spanning-tree stp <STG number> vlan <VLAN>
RS G8000(config)# vlan
<VLAN number>
RS G8000(config-vlan)# stg <STG number>
RS G8000(config-vlan)# exit
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