Cisco Nexus 5000 Series Switch CLI Software Configuration Guide, NX-OS 4.0(1a)N1 (OL-16597-01, January 2009)

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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 10 Configuring STP Extensions
Information About STP Extensions
Understanding Loop Guard
Loop Guard protects networks from loops that are caused by the following:
• Network interfaces that malfunction
• Busy CPUs
• Anything that prevents the normal forwarding of BPDUs
An STP loop occurs when a blocking port in a redundant topology erroneously transitions to the
forwarding state. This transition usually happens because one of the ports in a physically redundant
topology (not necessarily the blocking port) stops receiving BPDUs.
Loop Guard is only useful in switched networks where devices are connected by point-to-point links. On
a point-to-point link, a designated bridge cannot disappear unless it sends an inferior BPDU or brings
the link down.
Note Loop Guard can be enabled only on network and normal spanning tree port types.
You can use Loop Guard to determine if a root port or an alternate/backup root port receives BPDUs. If
the port does not receive BPDUs, Loop Guard puts the port into an inconsistent state (blocking) until the
port starts to receive BPDUs again. A port in the inconsistent state does not transmit BPDUs. If the port
receives BPDUs again, the protocol removes its loop-inconsistent condition, and the STP determines the
port state because such recovery is automatic.
Loop Guard isolates the failure and allows STP to converge to a stable topology without the failed link
or bridge. Disabling Loop Guard moves all loop-inconsistent ports to the listening state. (See Chapter 8,
“Configuring Rapid PVST+” for information on STP port states.)
You can enable Loop Guard on a per-port basis. When you enable Loop Guard on a port, it is
automatically applied to all of the active instances or VLANs to which that port belongs. When you
disable Loop Guard, it is disabled for the specified ports.
Table 10-1 BPDU Filtering Configurations
BPDU Filtering Per Port
Configuration
BPDU Filtering Global
Configuration
STP Edge Port
Configuration BPDU Filtering State
Default Enable Enable Enable
1
1. The port transmits at least 10 BPDUs. If this port receives any BPDUs, the port returns to the spanning tree normal port state
and BPDU Filtering is disabled.
Default Enable Disable Disable
Default Disable Not applicable Disable
Disable Not applicable Not applicable Disable
Enable Not applicable Not applicable Enable