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Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 12 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 12-6 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure 12-7, BackboneFast is
not activated because the inferior BPDUs did not come from the recognized designated bridge
(Switch B). The new switch begins sending inferior BPDUs that indicate it is the root switch. However,
the other switches ignore these inferior BPDUs, and the new switch learns that Switch B is the
designated bridge to Switch A, the root switch.
Figure 12-7 Adding a Switch in a Shared-Medium Topology
Understanding EtherChannel Guard
You can use EtherChannel guard to detect an EtherChannel misconfiguration between the switch and a
connected device. A misconfiguration can occur if the switch interfaces are configured in an
EtherChannel, but the interfaces on the other device are not. A misconfiguration can also occur if the
channel parameters are not the same at both ends of the EtherChannel. For EtherChannel configuration
guidelines, see the “EtherChannel Configuration Guidelines” section on page 25-8.
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Link failure
BackboneFast transitions port
through listening and learning
states to forwarding state.
87820
Switch A
(Root)
Switch C
Switch B
(Designated bridge)
Added switch
Blocked port
87821