Cisco Gigabit Ethernet Switch Module for HP p-Class BladeSystem Software Configuration Guide

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Chapter 14 Configuring Optional Spanning-Tree Features

Understanding Optional Spanning-Tree Features

Understanding Port Fast

Port Fast immediately brings an interface configured as an access or trunk port to the forwarding state

from a blocking state, bypassing the listening and learning states.

Port Fast is enabled by default on interfaces that are connected to a blade server (ports 1 to 16), as shown

in Figure 14-1, to allow those devices to immediately connect to the network, rather than waiting for the

spanning tree to converge.

Interfaces connected to a blade server should not receive bridge protocol data units (BPDUs). An

interface 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 interfaces connected to end stations. If you enable Port Fast

on an interface connecting to another switch, you risk creating a spanning-tree loop.

You can enable this feature by using the spanning-tree portfast interface configuration or the

spanning-tree portfast default global configuration command.

Figure 14-1 Port Fast-Enabled Interfaces

Understanding BPDU Guard

The BPDU guard feature can be globally enabled on the switch or can be enabled per port, but the feature

operates with some differences.

At the global level, you enable BPDU guard on Port Fast-enabled ports by using the spanning-tree

portfast bpduguard default global configuration command. Spanning tree shuts down ports that are in

a Port Fast-operational state if any BPDU is received on them. In a valid configuration, Port Fast-enabled

ports do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled port means an invalid

configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the

port in the error-disabled state. When this happens, the switch shuts down the entire port on which the

violation occurred.

Blade ServersBlade Servers

Port

Fast-enabled

ports

119646

Blade Switch