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16-2
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 16 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Understanding Port Fast
Port Fast immediately brings an STP port configured as an access or trunk port to the forwarding state
from a blocking state, bypassing the listening and learning states.
Note By default, STP is enabled on NNIs and disabled on ENIs. UNIs do not support STP. If a port is a UNI,
you can configure it as an STP port by changing the port type to NNI or ENI and entering the port-type
{nni | eni} interface configuration command. For ENIs, you then need to enter the spanning-tree
interface configuration command to configure the port as an STP port.
You can use Port Fast on STP ports connected to a single workstation or server, as shown in Figure 16-1,
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 16-4. 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 16-1 Port Fast-Enabled Interfaces
WorkstationsWorkstations
Server
Port
Fast-enabled port
Port
Fast-enabled
ports
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