Understanding Loop Guard; Configuring Optional Spanning-Tree Features - Cisco WS-C3560-48PS-S Software Configuration Manual

Understanding Optional Spanning-Tree Features
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 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.
If your switch is running PVST+, rapid PVST+, or MSTP, 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 17-8 Root Guard in a Service-Provider Network
spanning-tree root without
root guard enabled

Understanding Loop Guard

You can use loop guard to prevent alternate or root ports from becoming designated ports because of a
failure that leads to a unidirectional link. This feature is most effective when it is enabled on the entire
switched network. Loop guard prevents alternate and root ports from becoming designated ports, and
spanning tree does not send BPDUs on root or alternate ports.
If your switch is running PVST+, rapid PVST+, or MSTP, you can enable this feature by using the
spanning-tree loopguard default global configuration command.
When the switch is operating in PVST+ or rapid-PVST+ mode, loop guard prevents alternate and root
ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.
Catalyst 3560 Switch Software Configuration Guide
17-8
Customer network
Potential
Chapter 17

Configuring Optional Spanning-Tree 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.
Desired
root switch
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