Cisco WS-C2950SX-48-SI Configuration Manual page 163

Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard
Figure 8-6
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Figure 8-6
• Switches A and B are distribution switches.
• Switch C is an access switch.
• Loop guard is enabled on ports 3/1 and 3/2 on Switches A, B, and C.
Use loop guard only in topologies where there are blocked ports. Topologies that have no blocked ports,
which are loop free, do not need to enable this feature. Enabling loop guard on a root switch has no effect
but provides protection when a root switch becomes a nonroot switch.
Follow these guidelines when using loop guard:
Do not enable loop guard on PortFast-enabled or dynamic VLAN ports.
•
Do not enable PortFast on loop guard-enabled ports.
•
Do not enable loop guard if root guard is enabled.
•
Do not enable loop guard on ports that are connected to a shared link.
•
Note
Loop guard interacts with other features as follows:
• Loop guard does not affect the functionality of UplinkFast or BackboneFast.
• Root guard forces a port to always be designated as the root port. Loop guard is effective only if the
port is a root port or an alternate port. Do not enable loop guard and root guard on a port at the same
time.
• PortFast transitions a port into a forwarding state immediately when a link is established. Because a
PortFast-enabled port will not be a root port or alternate port, loop guard and PortFast cannot be
configured on the same port. Assigning dynamic VLAN membership for the port requires that the port
is PortFast enabled. Do not configure a loop guard-enabled port with dynamic VLAN membership.
If your network has a type-inconsistent port or a PVID-inconsistent port, all BPDUs are dropped
•
until the misconfiguration is corrected. The port transitions out of the inconsistent state after the
message age expires. Loop guard ignores the message age expiration on type-inconsistent ports and
78-15486-01
Triangle Switch Configuration with Loop Guard
A
3/1 3/1
3/1 3/2
C
Designated port
Root port
Alternate port
illustrates the following configuration:
We recommend that you enable loop guard on root ports and alternate root ports on access
switches.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
B
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Understanding How Loop Guard Works
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Release 8.1
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