Understanding Loop Guard; Understanding Root Guard - Cisco Nexus 3000 series Configuration Manual

Information About STP Extensions
BPDU Filtering Per Port
Configuration
Enable

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.
Loop Guard can be enabled only on network and normal spanning tree port types.
Note
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.
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.

Understanding 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 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)
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BPDU Filtering Global STP Edge Port
Configuration Configuration
Enabled/Disabled Enabled/Disabled
Configuring STP Extensions
BPDU Filtering State
Enable
BPDUs are
Caution
never sent and
if received,
they do not
trigger the
regular STP
behavior - use
with caution.