Understanding Root Guard - Cisco Catalyst 3750 Software Configuration Manual

Chapter 16 Configuring Optional Spanning-Tree Features
Figure 16-6 BackboneFast Example After Indirect Link Failure
Switch A
(Root)
If a new switch is introduced into a shared-medium topology as shown in
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 16-7 Adding a Switch in a Shared-Medium Topology
Switch C

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
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.
78-15870-01
L1
Link failure
L2
BackboneFast changes port
through listening and learning
states to forwarding state.
Switch C
Switch A
(Root)
Blocked port
Figure
Understanding Optional Spanning-Tree Features
Switch B
L3
Switch B
(Designated bridge)
Added switch
16-8. You can avoid this situation by enabling root guard on SP
Catalyst 3750 Metro Switch Software Configuration Guide
Figure 16-7, BackboneFast is
16-7