Cisco Systems 2960 Understanding EtherChannel Guard

17-7
Catalyst 2960 Switch SoftwareConfiguration Guide
78-16881-01
Chapter17 Configuring Optiona l Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure17-6 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure17-7, BackboneFast is
not activated because the inferior BPDUs did not come from the re cognized designated switch
(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 switch to SwitchA, the root switch.
Figure17-7 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 th e EtherChannel. For EtherChannel configuration
guidelines, see the “EtherChannel Configuration Guidelines” section on page 30-9.
If the switch detects a misconfiguration on the other device, EtherChannel guard places the switch
interfaces in the error-disabled state, and displays an error message.
You can enable this feature by using the spanning-tree etherchannel guard misconfig global
configuration command.
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Link failure
44964
BackboneFast changes port
through listening and learning
states to forwarding state.
Switch A
(Root)
Switch C Switch B
(Designated bridge)
Added switch
44965
Blocked port