NEC INTELLIGENT L2 SWITCH N8406-022A - Manual (Page 51)

Spanning Tree Protocol 51
Adding and removing ports from STGs
Information on adding and removing ports from STGs is as follows:
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By default, all ports except Port 19 belong to VLAN 1 and STG 1.
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Each port is always a member of at least one VLAN. Each VLAN is always a member of at least one STG.
Port membership within VLANs can be changed, and VLAN membership within STGs can be changed. To
move a port from one STG to another, move the VLAN to which the port belongs, or move the port to a VLAN
that belongs to the STG.
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When you remove a port from a VLAN, that port is also removed from the STG to which the VLAN belongs.
However, if that port belongs to another VLAN in the same STG, the port remains in the STG.
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If you remove an untagged port from a non-default VLAN and STG, it is added to VLAN 1 and STG 1.
The relationship between ports, trunk groups, VLANs, and spanning trees is shown in the following table.
Table 10 Ports, trunk groups, and VLANs
Switch element Belongs to
Port Trunk group, or one or more VLANs
Trunk group One or more VLANs
VLAN (non-default) One Spanning Tree Group
Assigning cost to ports and trunk groups
When you configure a trunk group to participate in a Spanning Tree Group, manually assign port and trunk costs to
ensure that each trunk group has a lower STP cost than the cost of each port within the trunk group. This ensures
that the trunk group remains in the Forwarding state.
Multiple Spanning Trees
Each switch supports a maximum of 32 Spanning Tree Groups (STGs). Multiple STGs provide multiple data paths,
which can be used for load-balancing and redundancy.
You enable independent links on two switches using multiple STGs by configuring each path with a different VLAN
and then assigning each VLAN to a separate STG. Each STG is independent. Each STG sends its own Bridge
Protocol Data Units (BPDUs), and each STG must be independently configured.
The STG, or bridge group, forms a loop-free topology that includes one or more virtual LANs (VLANs). The switch
supports 32 STGs running simultaneously. The default STG 1 supports IEEE 802.1d Spanning Tree Protocol, and
may contain more than one VLAN. All other STGs support Per VLAN Spanning Tree (PVST+), and may contain
only one VLAN each. The switch can support multiple VLANs in STGs 2-32; however, you must enable IEEE
802.1s Multiple Spanning Tree Protocol mode. For more information, see the “RSTP and MSTP” chapter in this
guide.
Why do we need Multiple Spanning Trees?
The following figure shows a simple example of why we need multiple Spanning Trees. This example assumes that
port 17 and 18 are not part of Trunk Group 1. Two VLANs (VLAN 1 and VLAN 2) exist between Switch 1 and
Switch 2. If the same Spanning Tree Group is enabled on both switches, the switches see an apparent loop and
block port 18 on Switch 2, which cuts off communication between the switches for VLAN 2.