Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual page 1907

Example: Configuring Loop Protection to Prevent Interfaces from Transitioning from
Blocking to Forwarding in a Spanning Tree on EX Series Switches
Requirements
Overview and Topology
Copyright © 2010, Juniper Networks, Inc.
Example: Configuring Root Protection to Enforce Root Bridge Placement in Spanning
Trees on EX Series Switches on page 1807
Understanding BPDU Protection for STP, RSTP, and MSTP on EX Series Switches on
page 1756
EX Series switches provide Layer 2 loop prevention through Spanning Tree Protocol
(STP), Rapid Spanning Tree protocol (RSTP), and Multiple Spanning Tree Protocol
(MSTP). Loop protection increases the efficiency of STP, RSTP, and MSTP by preventing
interfaces from moving into a forwarding state that would result in a loop opening up in
the network.
This example describes how to configure loop protection for an interface on an EX Series
switch in an RSTP topology:
Requirements on page 1803
Overview and Topology on page 1803
Configuration on page 1805
Verification on page 1805
This example uses the following hardware and software components:
Junos OS Release 9.1 or later for EX Series switches
Three EX Series switches in an RSTP topology
Before you configure the interface for loop protection, be sure you have:
RSTP operating on the switches.
NOTE: By default, RSTP is enabled on all EX Series switches.
A loop-free network in spanning-tree topologies is supported through the exchange of
a special type of frame called bridge protocol data unit (BPDU). Peer STP applications
running on the switch interfaces use BPDUs to communicate. Ultimately, the exchange
of BPDUs determines which interfaces block traffic (preventing loops) and which
interfaces become root ports and forward traffic.
A blocking interface can transition to the forwarding state in error if the interface stops
receiving BPDUs from its designated port on the segment. Such a transition error can
occur when there is a hardware error on the switch or software configuration error between
the switch and its neighbor. When this happens, a loop opens up in the spanning tree.
Chapter 71: Examples of Spanning-Tree Protocols Configuration
1803