Juniper Networks EX2500 manual Bridge Protocol Data Units BPDUs, Bridge Priority

EX2500 Ethernet Switch Configuration Guide

The relationship between port, trunk groups, VLANs, and spanning trees is shown in Table 9.

Table 9: Ports, Trunk Groups, and VLANs

Switch Element	Belongs to
Port	Trunk group
	or
	One or more VLANs
Trunk group	One or more VLANs
VLAN (non-default)	RSTP: All VLANs in STG 1
	PVRST+: One VLAN per Spanning Tree Group
	MSTP: Multiple VLANs per Spanning Tree Group

NOTE: Due to Spanning Tree’s sequence of discarding, learning, and forwarding, lengthy delays might occur. You can use a port’s spanning-tree edge command to permit a port that participates in Spanning Tree to bypass the Discarding and Learning states, and enter directly into the Forwarding state.

Bridge Protocol Data Units (BPDUs)

To create a spanning tree, the 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 2 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 MAC address, bridge priority, port priority, and path cost.

The generic action of a switch upon receiving a BPDU is to compare the received BPDU to its own BPDU that it will transmit. If the received BPDU is better than its own BPDU, it will replace its BPDU with the received BPDU. Then, the switch uses this information to block any necessary ports.

Determining the Path for Forwarding BPDUs

When determining which port to use for forwarding and which port to block, the EX2500 switch uses information in the BPDU, including each bridge 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 STG root bridge. To make one switch become 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. Use the following command to configure the spanning-tree bridge priority:

ex2500(config)# spanning-tree stp 1 bridge priority <0-61440, in steps of 4096>

32„ Spanning Tree Overview