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Alteon OS Application Guide
Chapter 5: Spanning Tree Group
10942C4911, January 2007
If ports are tagged, all trunked ports can belong to multiple STGs.
A port that is not a member of any VLAN cannot be added to any STG. The port must be
added to a VLAN, and that VLAN added to the desired STG.
Rules for VLAN Tagged ports
Tagged ports can belong to more than one STG, but untagged ports can belong to only one
STG.
When a tagged port belongs to more than one STG, the egress BPDUs are tagged to distin-
guish the BPDUs of one STG from those of another STG.
An untagged port cannot span multiple STGs.
Adding and removing ports from STGs
When you add a port to a VLAN that belongs to an STG, the port is also added to the STG.
However, if the port you are adding is an untagged port and is already a member of an
STG, that port will not be added to an additional STG because an untagged port cannot
belong to more that one STG.
For example, assume that VLAN 1 belongs to STG 1. You add an untagged port, port 1,
that does not belong to any STG to VLAN 1, and port 1 will become part of STG 1.
If you add untagged port 5 (which is a member to STG 2) to STG 1, the switch will
prompt you to change the PVID from 2 to 1:
When you remove a port from VLAN that belongs to an STG, that port will also be
removed from the STG. However, if that port belongs to another VLAN in the same STG,
the port remains in the STG.
As an example, assume that port 1 belongs to VLAN 1, and VLAN 1 belongs to STG 1.
When you remove port 1 from VLAN 1, port 1 is also removed from STG 1.
However, if port 1 belongs to both VLAN 1 and VLAN 2 and both VLANs belong to
STG 1, removing port 1 from VLAN 1 does not remove port 1 from STG 1 because
VLAN 2 is still a member of STG 1.
An STG cannot be deleted, only disabled. If you disable the STG while it still contains
VLAN members, Spanning Tree will be off on all ports belonging to that VLAN.
The relationship between port, trunk groups, VLANs, and Spanning Trees is shown in
Table 5-1.
"Port 5 is an UNTAGGED port and its current PVID is 2.
Confirm changing PVID from 2 to 1 [y/n]:" y