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Catalyst 3560 Switch Software Configuration Guide
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Chapter 17 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 17-6 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure 17-7, BackboneFast is
not activated because the inferior BPDUs did not come from the recognized 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 Switch A, the root switch.
Figure 17-7 Adding a Switch in a Shared-Medium Topology
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 17-8. 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 places 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.
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
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