Understanding Root Guard - Cisco Catalyst 2360 Software Configuration Manual

Understanding Optional Spanning-Tree Features

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.
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) instance because of root
guard, the interface also is blocked in all MST instances. A boundary port is an interface that connects
to a LAN, the designated switch of which is either an IEEE 802.1D switch or a switch with a different
MST region configuration.
Root guard enabled on an interface applies to all the VLANs to which the interface belongs. VLANs can
be grouped and mapped to an MST instance.
You can enable this feature by using the spanning-tree guard root interface configuration command.
Caution Misuse of the root-guard feature can cause a loss of connectivity.
Figure 13-8
spanning-tree root without
root guard enabled
Catalyst 2360 Switch Software Configuration Guide
13-8
Figure
Root Guard in a Service-Provider Network
Customer network
Potential
Chapter 13
13-8. You can avoid this situation by enabling root guard on SP
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.
Configuring Optional Spanning-Tree Features
Desired
root switch
OL-19808-01