Juniper EX9200 Features Manual page 255

Requirements
Overview and Topology
Copyright © 2017, Juniper Networks, Inc.
This example describes how to configure root protection on an interface on an EX Series
switch:
Requirements on page 239
Overview and Topology on page 239
Configuration on page 241
Verification on page 241
This example uses the following software and hardware components:
Junos OS Release 13.2X50-D10 or later or later for EX Series switches
Four EX Series switches in an RSTP topology
Before you configure the interface for root protection, be sure you have:
RSTP operating on the switches.
NOTE: By default, RSTP is enabled on all EX Series switches.
Peer STP applications running on switch interfaces exchange a special type of frame
called a bridge protocol data unit (BPDU). Switches communicate interface information
using BPDUs to create a loop-free topology that ultimately determines the root bridge
and which interfaces block or forward traffic in the spanning tree.
However, a root port elected through this process has the possibility of being wrongly
elected. A user bridge application running on a PC can generate BPDUs, too, and interfere
with root port election.
To prevent this from happening, enable root protection on interfaces that must not receive
superior BPDUs from the root bridge and must not be elected as the root port. These
interfaces are typically located on an administrative boundary and are designated ports.
When root protection is enabled on an interface:
The interface is blocked from becoming the root port.
Root protection is enabled for all STP instances on that interface.
The interface is blocked only for instances for which it receives superior BPDUs.
Otherwise, it participates in the spanning-tree topology.
CAUTION: An interface can be configured for either root protection or loop
protection, but not for both.
Chapter 10: Using BPDU Protection, Root Protection, and Loop Protection
239