Juniper Networks EX2500 - Manual (Page 48)

EX2500 Ethernet Switch Configuration Guide
34 „ Spanning Tree Overview
The relationship between port, trunk groups, VLANs, and spanning trees is shown
in Table 9.
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>
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.