NEC INTELLIGENT L2 SWITCH N8406-022A - Manual (Page 49)

Spanning Tree Protocol 49
Spanning Tree Protocol
Introduction
When multiple paths exist on a network, Spanning Tree Protocol (STP) configures the network so that a switch
uses only the most efficient path. The following topics are discussed in this chapter:
• Overview
•
Bridge Protocol Data Units (BPDUs)
•
Spanning Tree Group (STG) configuration guidelines
•
Multiple Spanning Trees
Overview
Spanning Tree Protocol (STP) detects and eliminates logical loops in a bridged or switched network. STP forces
redundant data paths into a standby (blocked) state. When multiple paths exist, STP configures the network so that
a switch uses only the most efficient path. If that path fails, STP automatically sets up another active path on the
network to sustain network operations.
The switch supports IEEE 802.1D Spanning Tree Protocol for STG 1, and Per VLAN Spanning Tree Protocol
(PVST+) for STGs 2-32, by default.
NOTE: The switch also supports IEEE 802.1w Rapid Spanning Tree Protocol and IEEE 802.1s Multiple
Spanning Tree Protocol. For more information, see the “RSTP and MSTP” chapter in this guide.
Bridge Protocol Data Units
To create a spanning tree, the application switch generates a configuration Bridge Protocol Data Unit (BPDU),
which it then forwards out of its ports. All switches in the Layer 2 network participating in the spanning tree gather
information about other switches in the network through an exchange of BPDUs.
A BPDU is a 64-byte packet that is sent out at a configurable interval, which is typically set for two seconds. The
BPDU is used to establish a path, much like a “hello” packet in IP routing. BPDUs contain information about the
transmitting bridge and its ports, including bridge and MAC addresses, bridge priority, port priority, and port path
cost. If the ports are tagged, each port sends out a special BPDU containing the tagged information.
The generic action of a switch on receiving a BPDU is to compare the received BPDU to its own BPDU that it will
transmit. If the received BPDU has a priority value closer to zero than its own BPDU, it will replace its BPDU with
the received BPDU. Then, the application switch adds its own bridge ID number and increments the path cost of
the BPDU. The application switch uses this information to block any redundant paths.
Determining the path for forwarding BPDUs
When determining which port to use for forwarding and which port to block, the switch uses information in the
BPDU, including each bridge priority ID. A technique based on the “lowest root cost” is then computed to determine
the most efficient path for forwarding.
Bridge priority
The bridge priority parameter controls which bridge on the network is the STP root bridge. To make one switch the
root bridge, configure the bridge priority lower than all other switches and bridges on your network. The lower the
value, the higher the bridge priority. The bridge priority is configured using the /cfg/l2/stp/brg/prior
command in the CLI.
Port priority
The port priority helps determine which bridge port becomes the designated port. In a network topology that has
multiple bridge ports connected to a single segment, the port with the lowest port priority becomes the designated
port for the segment. The port priority is configured using the /cfg/l2/stp/port x/prior command in the CLI.