Juniper EX9200 Series Specification sheet

Spanning-Tree Protocols Feature Guide for
EX9200 Switches
Modified: 2017-07-12
Copyright © 2017, Juniper Networks, Inc.

Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, California 94089
USA
408-745-2000
www.juniper.net
Juniper Networks, Junos, Steel-Belted Radius, NetScreen, and ScreenOS are registered trademarks of Juniper Networks, Inc. in the United
States and other countries. The Juniper Networks Logo, the Junos logo, and JunosE are trademarks of Juniper Networks, Inc. All other
trademarks, service marks, registered trademarks, or registered service marks are the property of their respective owners.
Juniper Networks assumes no responsibility for any inaccuracies in this document. Juniper Networks reserves the right to change, modify,
transfer, or otherwise revise this publication without notice.
Spanning-Tree Protocols Feature Guide for EX9200 Switches
Copyright © 2017, Juniper Networks, Inc.
All rights reserved.
The information in this document is current as of the date on the title page.
YEAR 2000 NOTICE
Juniper Networks hardware and software products are Year 2000 compliant. Junos OS has no known time-related limitations through the
year 2038. However, the NTP application is known to have some difficulty in the year 2036.
END USER LICENSE AGREEMENT
The Juniper Networks product that is the subject of this technical documentation consists of (or is intended for use with) Juniper Networks
software. Use of such software is subject to the terms and conditions of the End User License Agreement (“EULA”) posted at
http://www.juniper.net/support/eula.html. By downloading, installing or using such software, you agree to the terms and conditions of
that EULA.
Copyright © 2017, Juniper Networks, Inc.ii

Table of Contents
AbouttheDocumentation............................................xi
DocumentationandReleaseNotes..................................xi
SupportedPlatforms.............................................xi
UsingtheExamplesinThisManual..................................xi
MergingaFullExample.......................................xii
MergingaSnippet............................................xii
DocumentationConventions......................................xiii
DocumentationFeedback.........................................xv
RequestingTechnicalSupport.....................................xv
Self-Help Online Tools and Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
OpeningaCasewithJTAC.....................................xvi
Part 1 RSTP
Chapter1 UsingRSTP........................................................3
Understanding RSTP for EX Series and QFX Series Switches . . . . . . . . . . . . . . . . . 3
Spanning Tree Protocols Help Prevent Broadcast Storms . . . . . . . . . . . . . . . . 4
RSTP is an Enhancement of the Original STP . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Port Roles Determine Participation in the Spanning Tree . . . . . . . . . . . . . . . . . 4
Port States Determine How a Port Processes a Frame . . . . . . . . . . . . . . . . . . . 5
Edge Ports Connect to Devices That Cannot Be Part of a Spanning Tree . . . . 5
BPDUsMaintaintheSpanning-Tree.................................5
WhenanRSTPRootBridgeFails....................................6
Switches Must Relearn MAC Addresses After a Link Failure . . . . . . . . . . . . . . . 6
SelectingaSpanningTreeProtocol..................................6
RSTP or VSTP Forced to Run as IEEE 802.1D STP . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Configuring a Spanning-Tree Instance Interface as an Edge Port for Faster
Convergence ....................................................8
Example: Configuring Faster Convergence and Improved Network Stability with
RSTP ..........................................................9
Forcing RSTP or VSTP to Run as IEEE 802.1D STP (CLI Procedure) . . . . . . . . . . . 27
Configuring Spanning Tree Protocols (J-Web Procedure) . . . . . . . . . . . . . . . . . . . 28
ConfiguringRSTP(CLIProcedure).....................................33
Reverting to RSTP or VSTP from Forced IEEE 802.1D STP . . . . . . . . . . . . . . . . . . . 34
Chapter 2 Configuration Statements for RSTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
cost..............................................................36
disable(SpanningTrees).............................................37
disable-timeout(SpanningTrees).....................................38
edge .............................................................39
force-version(IEEE802.1DSTP).......................................40
iiiCopyright © 2017, Juniper Networks, Inc.

forward-delay ......................................................41
hello-time.........................................................42
interface(BPDUBlocking)............................................43
interface(SpanningTree)............................................44
max-age..........................................................45
max-hops.........................................................46
mode.............................................................47
no-root-port.......................................................48
priority(ProtocolsSTP)..............................................49
protocols(STPType)...............................................50
revision-level.......................................................51
rstp ..............................................................52
traceoptions(SpanningTree).........................................54
Chapter3 RSTPMonitoringCommands.......................................57
MonitoringSpanningTreeProtocols....................................57
clear spanning-tree protocol-migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
clearspanning-treestatistics..........................................61
showspanning-treebridge...........................................62
showspanning-treeinterface.........................................67
showspanning-treestatistics.........................................73
Part 2 MSTP
Chapter4 Using MSTP ......................................................77
Understanding MSTP for EX Series and QFX Series Switches . . . . . . . . . . . . . . . . 78
MSTPMapsMultipleVLANs.......................................78
ConfiguringMSTPRegions........................................79
SelectingaSpanningTreeProtocol.................................79
Configuring a Spanning-Tree Instance Interface as an Edge Port for Faster
Convergence...................................................80
Example: Configuring Network Regions for VLANs with MSTP . . . . . . . . . . . . . . . 81
Configuring Spanning Tree Protocols (J-Web Procedure) . . . . . . . . . . . . . . . . . . 103
Configuring MSTP .................................................108
Chapter 5 Configuration Statements for MSTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
bridge-priority(SpanningTrees).......................................112
configuration-name.................................................113
cost..............................................................114
disable(SpanningTrees)............................................115
disable-timeout(SpanningTrees).....................................116
edge .............................................................117
force-version(IEEE802.1DSTP)......................................118
forward-delay .....................................................119
hello-time ........................................................120
interface(BPDUBlocking)...........................................121
interface(SpanningTree)............................................122
max-age .........................................................123
max-hops ........................................................124
mode ............................................................125
Copyright © 2017, Juniper Networks, Inc.iv
Spanning-Tree Protocols Feature Guide for EX9200 Switches

msti .............................................................126
mstp.............................................................127
no-root-port ......................................................129
priority(ProtocolsSTP).............................................130
protocols(STPType)...............................................131
revision-level ......................................................132
traceoptions(SpanningTree)........................................133
vlan(MSTP)......................................................136
vlan-group........................................................137
Chapter6 MSTPMonitoringCommands......................................139
MonitoringSpanningTreeProtocols...................................139
clear spanning-tree protocol-migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
clearspanning-treestatistics.........................................143
showspanning-treebridge..........................................144
showspanning-treeinterface........................................149
showspanning-treestatistics........................................155
Part 3 VSTP
Chapter7 UsingVSTP......................................................159
Understanding VSTP for EX Series Switches and QFX Series Switches . . . . . . . 159
Selecting a Spanning-Tree Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Configuring a Spanning-Tree Instance Interface as an Edge Port for Faster
Convergence...................................................161
RSTP or VSTP Forced to Run as IEEE 802.1D STP . . . . . . . . . . . . . . . . . . . . . . . . . 162
Forcing RSTP or VSTP to Run as IEEE 802.1D STP (CLI Procedure) . . . . . . . . . . . 162
Configuring Spanning Tree Protocols (J-Web Procedure) . . . . . . . . . . . . . . . . . . 163
Configuring VLAN Spanning Tree Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Reverting to RSTP or VSTP from Forced IEEE 802.1D STP . . . . . . . . . . . . . . . . . . 170
Chapter 8 Configuration Statements for VSTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
bridge-priority(SpanningTrees)......................................172
configuration-name ................................................173
cost .............................................................174
disable(SpanningTrees)............................................175
disable-timeout(SpanningTrees).....................................176
edge .............................................................177
forward-delay .....................................................178
hello-time ........................................................179
interface(BPDUBlocking)...........................................180
interface(SpanningTree)............................................181
max-age .........................................................182
max-hops ........................................................183
mode............................................................184
no-root-port ......................................................185
priority(ProtocolsSTP).............................................186
protocols(STPType)...............................................187
revision-level......................................................188
traceoptions(SpanningTree)........................................189
vCopyright © 2017, Juniper Networks, Inc.
Table of Contents

vlan(VSTP).......................................................192
vlan-group........................................................193
vstp .............................................................194
Chapter9 VSTPMonitoringCommands......................................197
MonitoringSpanningTreeProtocols...................................197
clear spanning-tree protocol-migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
clearspanning-treestatistics.........................................201
showspanning-treebridge..........................................202
showspanning-treeinterface........................................207
showspanning-treestatistics........................................213
Part 4 BPDU Protection, Root Protection, and Loop Protection
Chapter 10 Using BPDU Protection, Root Protection, and Loop Protection . . . . . . . . . 217
Understanding BPDU Protection for STP, RSTP, and MSTP on EX Series
Switches......................................................217
DifferentKindsofBPDUs........................................218
Protecting Switches from Incompatible BPDUs . . . . . . . . . . . . . . . . . . . . . . . 218
Example: Configuring BPDU Protection on Edge Interfaces to Prevent STP
Miscalculations ................................................219
Example: Configuring BPDU Protection on Interfaces to Prevent STP
Miscalculations on EX Series Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
Configuring BPDU Protection on Spanning Tree Interfaces . . . . . . . . . . . . . . . . . 230
Unblocking an Interface That Receives BPDUs in Error (CLI Procedure) . . . . . . . 231
Understanding Loop Protection for STP, RSTP, VSTP, and MSTP on EX Series
Switches......................................................231
Understanding Root Protection for STP, RSTP, VSTP, and MSTP on EX Series
Switches .....................................................233
Example: Configuring Loop Protection to Prevent Interfaces from Transitioning
from Blocking to Forwarding in a Spanning Tree on EX Series Switches . . . 234
Example: Configuring Root Protection to Enforce Root Bridge Placement in
Spanning Trees on EX Series Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
Chapter 11 Configuration Statements for BPDU Protection, Root Protection, and
Loop Protection .................................................245
bpdu-block.......................................................245
bpdu-block-on-edge...............................................246
bpdu-timeout-action...............................................247
Chapter 12 BPDU Protection, Root Protection, and Loop Protection Monitoring
Commands .....................................................249
clearerrorbpduinterface...........................................250
Copyright © 2017, Juniper Networks, Inc.vi
Spanning-Tree Protocols Feature Guide for EX9200 Switches

List of Figures
Part 1 RSTP
Chapter1 UsingRSTP........................................................3
Figure1:NetworkTopologyforRSTP....................................10
Part 2 MSTP
Chapter4 Using MSTP ......................................................77
Figure2:NetworkTopologyforMSTP...................................83
Part 4 BPDU Protection, Root Protection, and Loop Protection
Chapter 10 Using BPDU Protection, Root Protection, and Loop Protection . . . . . . . . . 217
Figure3:BPDUProtectionTopology...................................220
Figure4:BPDUProtectionTopology...................................226
Figure 5: Network Topology for Loop Protection . . . . . . . . . . . . . . . . . . . . . . . . . . 235
Figure 6: Network Topology for Root Protection . . . . . . . . . . . . . . . . . . . . . . . . . 240
viiCopyright © 2017, Juniper Networks, Inc.

Copyright © 2017, Juniper Networks, Inc.viii
Spanning-Tree Protocols Feature Guide for EX9200 Switches

List of Tables
AbouttheDocumentation..........................................xi
Table1:NoticeIcons.................................................xiii
Table 2: Text and Syntax Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv
Part 1 RSTP
Chapter1 UsingRSTP........................................................3
Table 3: Selecting a Spanning Tree Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table 4: Components of the Topology for Configuring RSTP . . . . . . . . . . . . . . . . . . 11
Table 5: Spanning-Tree Protocol Configuration Parameters . . . . . . . . . . . . . . . . . 29
Chapter3 RSTPMonitoringCommands.......................................57
Table 6: Summary of Spanning Tree Protocols Output Fields . . . . . . . . . . . . . . . . 57
Table 7: show spanning-tree bridge Output Fields . . . . . . . . . . . . . . . . . . . . . . . . . 63
Table 8: show spanning-tree Interface Output Fields . . . . . . . . . . . . . . . . . . . . . . 68
Table 9: show spanning-tree statistics Output Fields . . . . . . . . . . . . . . . . . . . . . . . 73
Part 2 MSTP
Chapter4 Using MSTP ......................................................77
Table 10: Selecting a Spanning Tree Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Table 11: Components of the Topology for Configuring MSTP on EX Series
Switches ......................................................83
Table 12: Spanning-Tree Protocol Configuration Parameters . . . . . . . . . . . . . . . . 104
Chapter6 MSTPMonitoringCommands......................................139
Table 13: Summary of Spanning Tree Protocols Output Fields . . . . . . . . . . . . . . . 139
Table 14: show spanning-tree bridge Output Fields . . . . . . . . . . . . . . . . . . . . . . . 145
Table 15: show spanning-tree Interface Output Fields . . . . . . . . . . . . . . . . . . . . . 150
Table 16: show spanning-tree statistics Output Fields . . . . . . . . . . . . . . . . . . . . . 155
Part 3 VSTP
Chapter7 UsingVSTP......................................................159
Table 17: Selecting a Spanning-Tree Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Table 18: Spanning-Tree Protocol Configuration Parameters . . . . . . . . . . . . . . . . 164
Chapter9 VSTPMonitoringCommands......................................197
Table 19: Summary of Spanning Tree Protocols Output Fields . . . . . . . . . . . . . . . 197
Table 20: show spanning-tree bridge Output Fields . . . . . . . . . . . . . . . . . . . . . . 203
Table 21: show spanning-tree Interface Output Fields . . . . . . . . . . . . . . . . . . . . . 208
Table 22: show spanning-tree statistics Output Fields . . . . . . . . . . . . . . . . . . . . . 213
ixCopyright © 2017, Juniper Networks, Inc.

Part 4 BPDU Protection, Root Protection, and Loop Protection
Chapter 10 Using BPDU Protection, Root Protection, and Loop Protection . . . . . . . . . 217
Table 23: Components of the Topology for Configuring BPDU Protection on EX
SeriesSwitches................................................221
Table 24: Components of the Topology for Configuring BPDU Protection on EX
SeriesSwitches................................................226
Table 25: Components of the Topology for Configuring Loop Protection on EX
SeriesSwitches................................................235
Table 26: Components of the Topology for Configuring Root Protection on EX
Series Switches ...............................................240
Copyright © 2017, Juniper Networks, Inc.x
Spanning-Tree Protocols Feature Guide for EX9200 Switches

About the Documentation
•Documentation and Release Notes on page xi
•Supported Platforms on page xi
•Using the Examples in This Manual on page xi
•Documentation Conventions on page xiii
•Documentation Feedback on page xv
•Requesting Technical Support on page xv
Documentation and Release Notes
To obtain the most current version of all Juniper Networks®technical documentation,
see the product documentation page on the Juniper Networks website at
http://www.juniper.net/techpubs/.
If the information in the latest release notes differs from the information in the
documentation, follow the product Release Notes.
Juniper Networks Books publishes books by Juniper Networks engineers and subject
matter experts. These books go beyond the technical documentation to explore the
nuances of network architecture, deployment, and administration. The current list can
be viewed at http://www.juniper.net/books.
Supported Platforms
For the features described in this document, the following platforms are supported:
•EX Series
Using the Examples in This Manual
If you want to use the examples in this manual, you can use the load merge or the load
merge relative command. These commands cause the software to merge the incoming
configuration into the current candidate configuration. The example does not become
active until you commit the candidate configuration.
If the example configuration contains the top level of the hierarchy (or multiple
hierarchies), the example is a full example. In this case, use the load merge command.
xiCopyright © 2017, Juniper Networks, Inc.

If the example configuration does not start at the top level of the hierarchy, the example
isasnippet. In this case, use the load merge relative command. These procedures are
described in the following sections.
Merging a Full Example
To merge a full example, follow these steps:
1. From the HTML or PDF version of the manual, copy a configuration example into a
text file, save the file with a name, and copy the file to a directory on your routing
platform.
For example, copy the following configuration to a file and name the file ex-script.conf.
Copy the ex-script.conf file to the /var/tmp directory on your routing platform.
system {
scripts {
commit {
file ex-script.xsl;
}
}
}
interfaces {
fxp0 {
disable;
unit 0 {
family inet {
address 10.0.0.1/24;
}
}
}
}
2. Merge the contents of the file into your routing platform configuration by issuing the
load merge configuration mode command:
[edit]
user@host# load merge /var/tmp/ex-script.conf
load complete
Merging a Snippet
To merge a snippet, follow these steps:
1. From the HTML or PDF version of the manual, copy a configuration snippet into a text
file, save the file with a name, and copy the file to a directory on your routing platform.
For example, copy the following snippet to a file and name the file
ex-script-snippet.conf. Copy the ex-script-snippet.conf file to the /var/tmp directory
on your routing platform.
commit {
file ex-script-snippet.xsl; }
Copyright © 2017, Juniper Networks, Inc.xii
Spanning-Tree Protocols Feature Guide for EX9200 Switches

2. Move to the hierarchy level that is relevant for this snippet by issuing the following
configuration mode command:
[edit]
user@host# edit system scripts
[edit system scripts]
3. Merge the contents of the file into your routing platform configuration by issuing the
load merge relative configuration mode command:
[edit system scripts]
user@host# load merge relative /var/tmp/ex-script-snippet.conf
load complete
For more information about the load command, see CLI Explorer.
Documentation Conventions
Table 1 on page xiii defines notice icons used in this guide.
Table 1: Notice Icons
DescriptionMeaningIcon
Indicates important features or instructions.Informational note
Indicates a situation that might result in loss of data or hardware damage.Caution
Alerts you to the risk of personal injury or death.Warning
Alerts you to the risk of personal injury from a laser.Laser warning
Indicates helpful information.Tip
Alerts you to a recommended use or implementation.Best practice
Table 2 on page xiv defines the text and syntax conventions used in this guide.
xiiiCopyright © 2017, Juniper Networks, Inc.
About the Documentation

Table 2: Text and Syntax Conventions
ExamplesDescriptionConvention
To enter configuration mode, type the
configure command:
user@host> configure
Represents text that you type.Bold text like this
user@host> show chassis alarms
No alarms currently active
Represents output that appears on the
terminal screen.
Fixed-width text like this
•A policy term is a named structure
that defines match conditions and
actions.
•Junos OS CLI User Guide
•RFC 1997, BGP Communities Attribute
•Introduces or emphasizes important
new terms.
•Identifies guide names.
•Identifies RFC and Internet draft titles.
Italic text like this
Configure the machine’s domain name:
[edit]
root@# set system domain-name
domain-name
Represents variables (options for which
you substitute a value) in commands or
configuration statements.
Italic text like this
•To configure a stub area, include the
stub statement at the [edit protocols
ospf area area-id] hierarchy level.
•The console port is labeled CONSOLE.
Represents names of configuration
statements, commands, files, and
directories; configuration hierarchy levels;
or labels on routing platform
components.
Text like this
stub <default-metric metric>;Encloses optional keywords or variables.< > (angle brackets)
broadcast | multicast
(string1 |string2 |string3)
Indicates a choice between the mutually
exclusive keywords or variables on either
side of the symbol. The set of choices is
often enclosed in parentheses for clarity.
| (pipe symbol)
rsvp { # Required for dynamic MPLS onlyIndicates a comment specified on the
same line as the configuration statement
to which it applies.
# (pound sign)
community name members [
community-ids ]
Encloses a variable for which you can
substitute one or more values.
[ ] (square brackets)
[edit]
routing-options {
static {
route default {
nexthop address;
retain;
}
}
}
Identifies a level in the configuration
hierarchy.
Indention and braces ( { } )
Identifies a leaf statement at a
configuration hierarchy level.
; (semicolon)
GUI Conventions
Copyright © 2017, Juniper Networks, Inc.xiv
Spanning-Tree Protocols Feature Guide for EX9200 Switches

Table 2: Text and Syntax Conventions (continued)
ExamplesDescriptionConvention
•In the Logical Interfaces box, select
All Interfaces.
•To cancel the configuration, click
Cancel.
Represents graphical user interface (GUI)
items you click or select.
Bold text like this
In the configuration editor hierarchy,
select Protocols>Ospf.
Separates levels in a hierarchy of menu
selections.
>(bold right angle bracket)
Documentation Feedback
We encourage you to provide feedback, comments, and suggestions so that we can
improve the documentation. You can provide feedback by using either of the following
methods:
•Online feedback rating system—On any page of the Juniper Networks TechLibrary site
at http://www.juniper.net/techpubs/index.html, simply click the stars to rate the content,
and use the pop-up form to provide us with information about your experience.
Alternately, you can use the online feedback form at
http://www.juniper.net/techpubs/feedback/.
•E-mail—Send your comments to [e-mail]. Include the document
or topic name, URL or page number, and software version (if applicable).
Requesting Technical Support
Technical product support is available through the Juniper Networks Technical Assistance
Center (JTAC). If you are a customer with an active J-Care or Partner Support Service
support contract, or are covered under warranty, and need post-sales technical support,
you can access our tools and resources online or open a case with JTAC.
•JTAC policies—For a complete understanding of our JTAC procedures and policies,
review the JTAC User Guide located at
http://www.juniper.net/us/en/local/pdf/resource-guides/7100059-en.pdf.
•Product warranties—For product warranty information, visit
http://www.juniper.net/support/warranty/.
•JTAC hours of operation—The JTAC centers have resources available 24 hours a day,
7 days a week, 365 days a year.
Self-Help Online Tools and Resources
For quick and easy problem resolution, Juniper Networks has designed an online
self-service portal called the Customer Support Center (CSC) that provides you with the
following features:
xvCopyright © 2017, Juniper Networks, Inc.
About the Documentation

•Find CSC offerings: http://www.juniper.net/customers/support/
•Search for known bugs: http://www2.juniper.net/kb/
•Find product documentation: http://www.juniper.net/techpubs/
•Find solutions and answer questions using our Knowledge Base: http://kb.juniper.net/
•Download the latest versions of software and review release notes:
http://www.juniper.net/customers/csc/software/
•Search technical bulletins for relevant hardware and software notifications:
http://kb.juniper.net/InfoCenter/
•Join and participate in the Juniper Networks Community Forum:
http://www.juniper.net/company/communities/
•Open a case online in the CSC Case Management tool: http://www.juniper.net/cm/
To verify service entitlement by product serial number, use our Serial Number Entitlement
(SNE) Tool: https://tools.juniper.net/SerialNumberEntitlementSearch/
Opening a Case with JTAC
You can open a case with JTAC on the Web or by telephone.
•Use the Case Management tool in the CSC at http://www.juniper.net/cm/.
•Call 1-888-314-JTAC (1-888-314-5822 toll-free in the USA, Canada, and Mexico).
For international or direct-dial options in countries without toll-free numbers, see
http://www.juniper.net/support/requesting-support.html.
Copyright © 2017, Juniper Networks, Inc.xvi
Spanning-Tree Protocols Feature Guide for EX9200 Switches

PART 1
RSTP
•Using RSTP on page 3
•Configuration Statements for RSTP on page 35
•RSTP Monitoring Commands on page 57
1Copyright © 2017, Juniper Networks, Inc.

Copyright © 2017, Juniper Networks, Inc.2
Spanning-Tree Protocols Feature Guide for EX9200 Switches

CHAPTER 1
Using RSTP
•Understanding RSTP for EX Series and QFX Series Switches on page 3
•RSTP or VSTP Forced to Run as IEEE 802.1D STP on page 8
•Configuring a Spanning-Tree Instance Interface as an Edge Port for Faster
Convergence on page 8
•Example: Configuring Faster Convergence and Improved Network Stability with
RSTP on page 9
•Forcing RSTP or VSTP to Run as IEEE 802.1D STP (CLI Procedure) on page 27
•Configuring Spanning Tree Protocols (J-Web Procedure) on page 28
•Configuring RSTP (CLI Procedure) on page 33
•Reverting to RSTP or VSTP from Forced IEEE 802.1D STP on page 34
Understanding RSTP for EX Series and QFX Series Switches
Ethernet networks are susceptible to broadcast storms if loops are introduced. However,
an Ethernet network needs to include loops because they provide redundant paths in
case of a link failure. Spanning-tree protocols address both of these issues because they
provide link redundancy while simultaneously preventing undesirable loops. Rapid
Spanning Tree Protocol (RSTP) is the default spanning-tree protocol for preventing
loops on Ethernet networks.
This topic describes:
•Spanning Tree Protocols Help Prevent Broadcast Storms on page 4
•RSTP is an Enhancement of the Original STP on page 4
•Port Roles Determine Participation in the Spanning Tree on page 4
•Port States Determine How a Port Processes a Frame on page 5
•Edge Ports Connect to Devices That Cannot Be Part of a Spanning Tree on page 5
•BPDUs Maintain the Spanning-Tree on page 5
•When an RSTP Root Bridge Fails on page 6
•Switches Must Relearn MAC Addresses After a Link Failure on page 6
•Selecting a Spanning Tree Protocol on page 6
3Copyright © 2017, Juniper Networks, Inc.

Spanning Tree Protocols Help Prevent Broadcast Storms
Spanning-tree protocols intelligently avoid loops in a network by creating a tree topology
(spanning tree) of the entire bridged network with only one available path between the
tree root and a leaf. All other paths are forced into a standby state. The tree root is a
switch within the network elected by the STA (spanning-tree algorithm) to use when
computing the best path between bridges throughout the network and the root bridge.
Frames travel through the network to their destination–a leaf such as an end-user
PC–along branches. A tree branch is a network segment, or link, between bridges. Switches
that forward frames through an STP spanning tree are called designated bridges.
Juniper Networks EX Series and QFX Series switches provide Layer 2 loop prevention
through Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP), Multiple
Spanning Tree Protocol (MSTP), and VLAN Spanning Tree Protocol (VSTP). This topic
explains the spanning-tree default RSTP.
NOTE: If you are using Junos OS for EX Series and QFX Series switches with
support for the Enhanced Layer 2 Software (ELS) configuration style, you
can force the original IEEE 802.1D Spanning Tree Protocol (STP) version to
run in place of RSTP or VSTP by setting force-version.
RSTP is an Enhancement of the Original STP
RSTP evolved from the original STP IEEE 802.1D protocol to provide faster spanning-tree
reconvergence after a switch port, switch, or LAN failure. Where STP took up to 50
seconds to respond to topology changes, RSTP responds to changes within the timeframe
of three hello BPDUs (bridge protocol data units), or 6 seconds. This is the primary reason
that RSTP is the default configuration on EX Series switches. In addition, note that EX
Series switches configured to use STP actually run RSTP force version 0, which is
compatible with STP.
Port Roles Determine Participation in the Spanning Tree
Each port has both a role and a state. A port’s role determines how it participates in the
spanning tree. The five port roles used in RSTP are:
•Root port—The port closest to the root bridge (has the lowest path cost from a bridge).
This is the only port that receives frames from and forwards frames to the root bridge.
•Designated port—The port that forwards traffic away from the root bridge toward a
leaf. A designated bridge has one designated port for every link connection it serves.
A root bridge forwards frames from all of its ports, which serve as designated ports.
•Alternate port—A port that provides an alternate path toward the root bridge if the
root port fails and is placed in the discarding state. This port is not part of the active
spanning tree, but if the root port fails, the alternate port immediately takes over.
•Backup port—A port that provides a backup path toward the leaves of the spanning
tree if a designated port fails and is placed in the discarding state. A backup port can
exist only where two or more bridge ports connect to the same LAN for which the bridge
Copyright © 2017, Juniper Networks, Inc.4
Spanning-Tree Protocols Feature Guide for EX9200 Switches