Understanding Root Guard - Cisco Catalyst 2950 Software Manual

Chapter 9 Configuring STP

Understanding Root Guard

The 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
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 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 network becomes the root switch, the interface is blocked (root-inconsistent state),
and STP selects a new root switch. The customer's switch does not become the root switch and is not in
the path to the root.
Misuse of the root-guard feature can cause a loss of connectivity.
Caution
Figure 9-13 STP in a Service Provider Network
STP root without
root guard enabled
78-11380-03
Figure 9-13. You can avoid this situation by configuring the root-guard feature on
Customer network
Potential
Understanding Advanced STP Features
Service-provider network
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.
Catalyst 2950 Desktop Switch Software Configuration Guide
Desired
root switch
9-19