Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual page 148

Understanding How Loop Guard Works
Loop guard interacts with other features as follows:
• Loop guard does not affect the functionality of UplinkFast or BackboneFast.
• Do not enable loop guard on ports that are connected to a shared link.
Note
Root guard forces a port to be always designated as the root port. Loop guard is effective only if the
•
port is a root port or an alternate port. You cannot 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. You cannot 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
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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
PVID-inconsistent ports. If the port is already blocked by loop guard, misconfigured BPDUs
received on the port make loop guard recover, but the port is moved into the type-inconsistent state
or PVID-inconsistent state.
• In high-availability switch configurations, if a port is put into the blocked state by loop guard, it
remains blocked even after switchover to the redundant supervisor engine. The newly activated
supervisor engine recovers the port only after receiving a BPDU on that port.
• Loop guard uses the ports known to spanning tree. Loop guard can take advantage of logical ports
provided by the Port Aggregation Protocol (PAgP). However, to form a channel, all the physical
ports grouped in the channel must have compatible configurations. PAgP enforces uniform
configurations of root guard or loop guard on all the physical ports to form a channel.
These caveats apply to loop guard:
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• Loop guard has no effect on a disabled spanning tree instance or a VLAN.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
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Chapter 8
We recommend that you enable loop guard on root ports and alternate root ports on access
switches.
Spanning tree always chooses the first operational port in the channel to send the BPDUs. If that
link becomes unidirectional, loop guard blocks the channel, even if other links in the channel
are functioning properly.
If a set of ports that are already blocked by loop guard are grouped together to form a channel,
spanning tree loses all the state information for those ports and the new channel port may obtain
the forwarding state with a designated role.
If a channel is blocked by loop guard and the channel breaks, spanning tree loses all the state
information. The individual physical ports may obtain the forwarding state with the designated
role, even if one or more of the links that formed the channel are unidirectional.
You can enable UniDirectional Link Detection (UDLD) to help isolate the link failure.
Note
A loop may occur until UDLD detects the failure, but loop guard will not be able to
detect it.
Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast, and Loop Guard
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