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184 Summit 200 Series Switch Installation and User Guide
Spanning Tree Protocol (STP)
The key points to remember when configuring VLANs and STP are:
• Each VLAN forms an independent broadcast domain
• STP blocks paths to create a loop-free environment
• When STP blocks a path, no data can be transmitted or received on the blocked port
• Within any given STPD, all VLANs belonging to it use the same spanning tree
• On a stacked configuration, a Spanning Tree for the network recognizes the stack as a single bridge.
The stacking ports do not run STP. However, a loop detected across the stacking links is cut by the
Stack Discovery protocol. Note that this configuration is not supported.
NOTE
Ensure that multiple STPD instances within a single switch do not see each other in the same
broadcast domain. This could happen if, for example, another external bridge is used to connect VLANs
belonging to separate STPDs.
If you delete an STPD, the VLANs that were members of that STPD are also deleted. You must remove
all VLANs associated with the STP before deleting the STPD.
Defaults
The default device configuration contains a single STPD called s0. The default VLAN is a member of
STPD s0.
All STP parameters default to the IEEE 802.1D values, as appropriate.
STPD BPDU Tunneling
You can configure ExtremeWare to allow a Bridge Protocol Data Unit (BPDU) to traverse a VLAN
without being processed by STP, even if STP is enabled on the port. This is known as BPDU tunneling.
To enable and disable BPDU tunneling on a VLAN, use the following command:
[enable | disable] ignore-bpdu vlan <name>
If you have a known topology and have switches outside of your network within your STPD, use this
feature to keep the root bridge within your network.
STP Configurations
When you assign VLANs to an STPD, pay careful attention to the STP configuration and its effect on
the forwarding of VLAN traffic.
The example network shown in Figure 31 uses VLAN tagging for trunk connections. The following four
VLANs have been defined:
• Sales is defined on switch A, switch B, and switch M.
• Personnel is defined on switch A, switch B, and switch M.
• Manufacturing is defined on switch Y, switch Z, and switch M.
• Engineering is defined on switch Y, switch Z, and switch M.