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Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 18 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 18-8 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 37-12.
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.
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 18-9. You can avoid this situation by enabling root guard on SP
switch interfaces that connect to switches in your customer’s network. If spanning-tree calculations
cause an interface in the customer network to be selected as the root port, root guard then plac es the
interface in the root-inconsistent (blocked) state to prevent the customer’s 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 customer’s switch does not become the root
switch and is not in the path to the root.
If the switch is operating in multiple spanning-tree (MST) mode, root guard forces the interface to be a
designated port. If a boundary port is blocked in an internal spanning-tree (IST) in stance because of root
guard, the interface also is blocked in all MST instances. A boundary port is an interface that connec ts
to a LAN, the designated switch of which is either an IEEE 802.1D switch or a switch with a different
MST region configuration.
Switch A
(Root)
Switch C Switch B
(Designated bridge)
Added switch
44965
Blocked port