Cisco WS-C2950-24 - Configuring STP Root Guard

Chapter 6 Configuring the System
Configuring STP
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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
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Configuring STP 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, STP can reconfigure
itself and select a customer switch as the STP root switch, as shown in Figure 6-9.
You can avoid this situation by configuring the root-guard feature on interfaces
that connect to switches outside of your customer’s network. If STP calculations
cause an interface in the customer network to be selected as the root port, root
guard then places the interface into the root-inconsistent (blocked) state to
prevent the customer switch from becoming the root switch or being in the path
to the root.
If a switch outside the network becomes the root switch, the interface is blocked
(root-inconsistent state), and STP selects a new root switch. The customer switch
does not become the root switch and is not in the path to the root.
Caution Misuse of this feature can cause a loss of connectivity.
Figure 6-9 STP in a Service Provider Network
Customer network
Potential
STP root without
root guard enabled
Enable the root-guard feature
on these interfaces to prevent
switches in the customer
network from becoming
the root switch or being
in the path to the root.
Desired
root switch
Service-provider network
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