Cisco Systems N3KC3048TP1GE, N3KC3064TFAL3 Understanding Loop Guard, Understanding Root Guard

Configuring STP Extensions

Information About STP Extensions

BPDU Filtering Per Port	BPDU Filtering Global	STP Edge Port	BPDU Filtering State
Configuration	Configuration	Configuration
Enable	Enabled/Disabled	Enabled/Disabled	Enable	BPDUs are
			Caution
				never sent and
				if received,
				they do not
				trigger the
				regular STP
				behavior - use
				with caution.

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.

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 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
116	OL-26590-01