Loop Guard; Root Guard - Cisco Nexus 7000 Series Configuration Manual

Loop Guard

Table 17: BPDU Filtering Configurations
BPDU Filtering Per Port
Configuration
Default
Default
Default
Disable
Enable
1 No explicit port configuration.
2 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.
Loop Guard
Loop Guard helps prevent bridging loops that could occur because of a unidirectional link failure on a
point-to-point link.
An STP loop occurs when a blocking port in a redundant topology erroneously transitions to the forwarding
state. Transitions are usually caused by a port in a physically redundant topology (not necessarily the blocking
port) that stops receiving BPDUs.
When you enable Loop Guard globally, it is useful only 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. However, you can enable Loop Guard on shared links per interface,
You can use Loop Guard to determine if a root port or an alternate/backup root port receives BPDUs. If the
port that was previously receiving BPDUs is no longer receiving BPDUs, Loop Guard puts the port into an
inconsistent state (blocking) until the port starts to receive BPDUs again. If such a port receives BPDUs again,
the port—and link—is deemed viable again. The protocol removes the loop-inconsistent condition from the
port, and the STP determines the port state because the 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.
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.
Enabling Loop Guard on a root device has no effect but provides protection when a root device becomes a
nonroot device.

Root Guard

When you enable Root Guard on a port, Root Guard does not allow that port to become a root port. If a
received BPDU triggers an STP convergence that makes that designated port become a root port, that port is
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
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BPDU Filtering Global
Configuration
1
Enable
Enable
Disable
Not applicable
Not applicable
Configuring STP Extensions Using Cisco NX-OS
STP Edge Port BPDU Filtering State
Configuration
Enable Enable
Disable Disable
Not applicable Disable
Not applicable Disable
Not applicable Enable
2