Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 11 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
You can use Port Fast on STP ports connected to a single workstation or server, as shown in Figure 11-1,
to allow those devices to immediately connect to the network, rather than waiting for the spanning tree
to converge.
STP ports connected to a single workstation or server should not receive bridge protocol data units
(BPDUs). An STP port with Port Fast enabled goes through the normal cycle of spanning-tree status
changes when the switch is restarted.
Note Because the purpose of Port Fast is to minimize the time interfaces must wait for spanning tree to
converge, it is effective only when used on STP ports connected to end stations. If you enable Port Fast
on an interface connecting to another switch, you risk creating a spanning-tree loop.
UNIs are typically customer-facing ports and do not participate in the spanning tree of the service
provider. However, if you configure a customer-facing port as an ENI and enable spanning tree, the ENI
could become the spanning tree root port unless you configure root guard on the port. See the
“Understanding Root Guard” section on page 11-3. A customer-facing ENI with STP enabled
participates in the same spanning tree as the service-provider facing NNI.
Note Exercise caution when enabling STP on a customer-facing ENI.
You can enable this feature by using the spanning-tree portfast interface configuration or the
spanning-tree portfast default global configuration command.
Figure 11-1 Port Fast-Enabled Interfaces
Understanding BPDU Guard
The BPDU guard feature can be globally enabled on the router or can be enabled per interface, but the
feature operates with some differences.
At the global level, you enable BPDU guard on Port Fast-enabled STP ports by using the spanning-tree
portfast bpduguard default global configuration command. Spanning tree shuts down STP ports that
are in a Port Fast-operational state if any BPDU is received on those ports. In a valid configuration, Port
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Fast-enabled port
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