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Cisco Catalyst 3550 Series User Manual

Cisco Catalyst 3550 Series
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18-12
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 18 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 18-9 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 30-8.
If the switch detects a misconfiguration on the other device, EtherChannel guard places the switch
interfaces in the error-disabled state, and this error message appears:
PM-4-ERR_DISABLE: Channel-misconfig error detected on [chars], putting [chars] in
err-disable state.
If your switch is running PVST+, rapid PVST+, or MSTP, you can enable this feature by using the
spanning-tree etherchannel guard misconfig global configuration command.
Understanding Root Guard
The Layer 2 network of a service provider (SP) can include many connections to switches that are not
owned by the SP. In such a topology, the spanning tree can reconfigure itself and select a customer switch
as the root switch, as shown in Figure 18-10. You can avoid this situation by enabling root guard on SP
switch interfaces that connect to switches in your customers network. If spanning-tree calculations
cause an interface in the customer network to be selected as the root port, root guard then places the
interface in the root-inconsistent (blocked) state to prevent the customers switch from becoming the root
switch or being in the path to the root.
If a switch outside the SP network becomes the root switch, the interface is blocked (root-inconsistent
state), and spanning tree selects a new root switch. The customers switch does not become the root
switch and is not in the path to the root.
Switch A
(Root)
Switch C
Switch B
(Designated bridge)
Added switch
44965
Blocked port

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Cisco Catalyst 3550 Series Specifications

General IconGeneral
BrandCisco
ModelCatalyst 3550 Series
CategoryNetwork Router
LanguageEnglish

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