IBM RackSwitch G8000, Application Manual

Page: 125 / 362

© Copyright IBM Corp. 2011 Chapter 10. Spanning Tree Protocols
123
Guidelines for Creating VLANs
Follow these guidelines when creating VLANs:
•
When you create a new VLAN, if VASA is enabled (the default), that VLAN is
automatically assigned its own STG. If VASA is disabled, the VLAN automatically
belongs to STG 1, the default STG. To place the VLAN in a different STG, see
“Manually Assigning STGs” on page 122 . The VLAN is automatically removed
from its old STG before being placed into the new STG.
• Each VLANs must be contained within a single STG; a VLAN cannot span
multiple STGs. By confining VLANs within a single STG, you avoid problems with
Spanning Tree blocking ports and causing a loss of connectivity within the VLAN.
When a VLAN spans multiple switches, it is recommended that the VLAN remain
within the same STG (be assigned the same STG ID) across all the switches.
• If ports are tagged, all trunked ports can belong to multiple STGs.
• A port cannot be directly added to an STG. The port must first be added to a
VLAN, and that VLAN added to the desired STG.
Rules for VLAN Tagged Ports
The following rules apply to 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 distinguish the BPDUs of one STG from those of another STG.
Adding and Removing Ports from STGs
The following rules apply when you add ports to or remove ports from STGs:
•
When you add a port to a VLAN that belongs to an STG, the port is also added to
that STG. However, if the port you are adding is an untagged port and is already
a member of another STG, that port will be removed from its current STG and
added to the new STG. An untagged port cannot belong to more that one STG.
For example: Assume that VLAN 1 belongs to STG 1, and that port 1 is
untagged and does not belong to any STG. When you add port 1 to VLAN 1, port
1 will automatically become part of STG 1.
However, if port 5 is untagged and is a member of VLAN 3 in STG 2, then adding
port 5 to VLAN 1 in STG 1 will change the port PVID from 3 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 2 belongs to only VLAN 2, and that VLAN 2
belongs to STG 2. When you remove port 2 from VLAN 2, the port is moved to
default VLAN 1 and is removed from STG 2.
However, if port 2 belongs to both VLAN 1 and VLAN 2, and both VLANs belong
to STG 2, removing port 2 from VLAN 2 does not remove port 2 from STG 1,
because the port is still a member of VLAN 1, which 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 13 on page 116 .
"Port 5 is an UNTAGGED port and its PVID changed from 3 to 1.