Lenovo ThinkSystem NE2552E, Application Manual

Page: 181 / 574

© Copyright Lenovo 2018 Chapter 10: Spanning Tree Protocols
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VLAN and STG Assignment
In PVRST mode, up to 128/256 STGs are supported, according to the existing
configuration. Ports cannot be added directly to an STG. Instead, ports must be
added as members of a VLAN, and the VLAN must then be assigned to the STG.
STG 1 is the default STG. Although VLANs can be added to or deleted from
default STG 1, the STG itself cannot be deleted from the system. By default, STG 1
is enabled and includes VLAN 1, which by default includes all switch ports (except
for management VLANs and management ports).
STG 256 is reserved for switch management. By default, STG 128/256 is disabled,
but includes management VLAN 4095 and the management ports.
By default, all other STGs (STG 2 through 127/255) 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 182 ).
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 127/255), the switch will attempt
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
command:
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.
NE2552E(config)# [ no ] spanning-tree stg-auto