189 copyright 2019 juniper networks inc chapter 5

189 Copyright © 2019, Juniper Networks, Inc.
Chapter 5: Loop Protection for Spanning-Tree Protocols

Action Use the operational mode command:
[email protected]> show spanning-tree interface
Spanning tree interface parameters for instance 0
Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ge-0/0/0.0 128:513 128:513 32768.0019e2503f00 20000 BLK DIS
ge-0/0/1.0 128:514 128:514 32768.0019e2503f00 20000 BLK DIS
ge-0/0/2.0 128:515 128:515 32768.0019e2503f00 20000 BLK DIS
ge-0/0/3.0 128:516 128:516 32768.0019e2503f00 20000 FWD DESG
ge-0/0/4.0 128:517 128:517 32768.0019e2503f00 20000 FWD DESG
ge-0/0/5.0 128:518 128:518 32768.0019e2503f00 20000 FWD DESG
ge-0/0/6.0 128:519 128:2 16384.00aabbcc0348 20000 BLK ALT
[output truncated]
Meaning The output from the operational mode command show spanning-tree interface shows
that ge-0/0/6.0 is the alternate port and in a blocking state.
Verifying That Loop Protection Is Working on an Interface
Purpose Verify the loop protection configuration on interface
ge-0/0/6
. RSTP has been disabled
on interface ge-0/0/4 on Switch 1. This will stop BPDUs from being sent to interface
ge-0/0/6 and trigger loop protection on the interface.
Action Use the operational mode command:
[email protected]> show spanning-tree interface
Spanning tree interface parameters for instance 0
Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ge-0/0/0.0 128:513 128:513 32768.0019e2503f00 20000 BLK DIS
ge-0/0/1.0 128:514 128:514 32768.0019e2503f00 20000 BLK DIS
ge-0/0/2.0 128:515 128:515 32768.0019e2503f00 20000 BLK DIS
ge-0/0/3.0 128:516 128:516 32768.0019e2503f00 20000 FWD DESG
ge-0/0/4.0 128:517 128:517 32768.0019e2503f00 20000 FWD DESG
ge-0/0/5.0 128:518 128:518 32768.0019e2503f00 20000 FWD DESG
ge-0/0/6.0 128:519 128:519 32768.0019e2503f00 20000 BLK DIS
(Loop-Incon)
[output truncated]
Meaning The operational mode command
show spanning-tree interface
shows that interface
ge-0/0/6.0
has detected that BPDUs are no longer being forwarded to it and has moved
into a loop-inconsistent state. The loop-inconsistent state prevents the interface from
transitioning to a forwarding state. The interface recovers and transitions back to its
original state as soon as it receives BPDUs.
Copyright © 2019, Juniper Networks, Inc. 190
Spanning-Tree Protocols Feature Guide

Example: Configuring Loop Protection to Prevent Interfaces from Transitioning from Blocking
to Forwarding in a Spanning Tree on EX Series Switches With ELS
NOTE: This example uses Junos OS for EX Series switches with support for
the Enhanced Layer 2 Software (ELS) configuration style. If your switch runs
software that does not support ELS, see “Example: Configuring Loop
Protection to Prevent Interfaces from Transitioning from Blocking to
Forwarding in a Spanning Tree on non-ELS EX Series Switches” on page 177 .
For ELS details, see Using the Enhanced Layer 2 Software CLI .
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.