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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter16 Configuring Optional Spanning-Tree Features
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) in stance because of root
guard, the interface also is blocked in all MST instances. A boundary port is an interface that connec ts
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.
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 16-2 Root Guard in a Service-Provider Network
Understanding Loop Guard
You can use loop guard to prevent alternate or root ports from becoming designated ports b eca use o f 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 port s.
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 gu ard prevents alter nate a nd root
ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.
When the switch is operating in MST mode, BPDUs are not sent o n n onboun da ry port s o nly if t he
interface is blocked by loop guard in all MST instances. On a bo undar y p ort, loo p g ua rd blocks t he
interface in all MST instances.
101232
Desired
root switch
Customer network
Potential
spanning-tree 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.
Service-provider network