Creating A Vlan; Rules For Vlan Tagged Ports; Adding And Removing Ports From Stgs - Juniper EX2500 Configuration Manual

EX2500 Ethernet Switch Configuration Guide
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Spanning Tree Overview
Each STG must have a VLAN assigned to it before it becomes functional. You
cannot configure other STG settings until the VLAN is assigned. If the STG VLAN
is unassigned, other configuration settings are cleared. Assign a VLAN and
reconfigure the STG settings.
NOTE: To ensure proper operation with switches that use Cisco Per VLAN
Spanning Tree (PVST+), you must either create a separate STG for each VLAN, or
manually add all associated VLANs into a single STG.

Creating a VLAN

When you create a VLAN, that VLAN automatically belongs to STG 1, the default
STG. You can assign the VLAN to another STG.
Move a newly created VLAN to an existing STG by following this order:
1. Create the VLAN.
2. Enable the VLAN.
3. Add the VLAN to an existing STG.
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, we recommend that the VLAN
remain within the same Spanning Tree Group (have the same STG ID) across all
the switches.
If ports are tagged, all tagged ports can belong to multiple STGs.
A port cannot be added directly to an STG. First add the port to a VLAN, then
add the VLAN to the STG.

Rules for VLAN Tagged Ports

Tagged ports can belong to more than one STG.
Untagged ports can belong to only one STG.

Adding and Removing Ports from STGs

When you add a port to a VLAN that belongs to an STG, the VLAN's member
port is 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 be removed from this
STG and added to the new STG. An untagged port cannot belong to more that
one STG.
For example, assume that VLAN 2 belongs to STG 2. You add an untagged port
(port 5) that belongs to STG 2 to VLAN 2. The port becomes a member of STG
2, and the switch displays a message to inform you that the PVID changed
from 1 to 2:
Port 5 is an UNTAGGED port and its PVID changed from 1 to 2.