Cisco Systems N3KC3048TP1GE manual Understanding Bridge Assurance, Understanding BPDU Guard

Configuring STP Extensions

Information About STP Extensions

Note If you configure a port connected to another switch as an edge port, you might create a bridging loop.

Spanning Tree Network Ports

Network ports are connected only to switches or bridges. Configuring a port as "network" while Bridge Assurance is enabled globally, enables Bridge Assurance on that port.

Note If you mistakenly configure ports that are connected to hosts or other edge devices, as spanning tree network ports, those ports will automatically move into the blocking state.

Spanning Tree Normal Ports

Normal ports can be connected to either hosts, switches, or bridges. These ports function as normal spanning tree ports.

The default spanning tree interface is a normal port.

Understanding Bridge Assurance

You can use Bridge Assurance to protect against certain problems that can cause bridging loops in the network. Specifically, you use Bridge Assurance to protect against a unidirectional link failure and a device that continues to forward data traffic when it is no longer running the spanning tree algorithm.

Note Bridge Assurance is supported only by Rapid PVST+ and MST. Legacy 802.1D spanning tree does not support Bridge Assurance.

Bridge Assurance is enabled by default and can only be disabled globally. Also, Bridge Assurance can be enabled only on spanning tree network ports that are point-to-point links. Finally, both ends of the link must have Bridge Assurance enabled.

With Bridge Assurance enabled, BPDUs are sent out on all operational network ports, including alternate and backup ports, for each hello time period. If the port does not receive a BPDU for a specified period, the port moves into the blocking state and is not used in the root port calculation. Once that port receives a BPDU, it resumes the normal spanning tree transitions.

Understanding BPDU Guard

Enabling BPDU Guard shuts down that interface if a BPDU is received.

You can configure BPDU Guard at the interface level. When configured at the interface level, BPDU Guard shuts the port down as soon as the port receives a BPDU, regardless of the port type configuration.

When you configure BPDU Guard globally, it is effective only on operational spanning tree edge ports. In a valid configuration, LAN edge interfaces do not receive BPDUs. A BPDU that is received by an edge LAN

	Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
114	OL-26590-01