Understanding Vstp For Ex Series Switches - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

Understanding Root Protection for STP, RSTP, VSTP, and MSTP on EX Series Switches
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Copyright © 2010, Juniper Networks, Inc.
Juniper Networks EX Series Ethernet Switches provide Layer 2 loop prevention through
Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP), VLAN Spanning
Tree Protocol (VSTP), and Multiple Spanning Tree Protocol (MSTP). A loop-free network
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 and which interfaces become root ports and forward traffic.
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. Root protection allows network administrators to manually
enforce the root bridge placement in the network.
Enable root protection on interfaces that should not receive superior BPDUs from the
root bridge and should not be elected as the root port. These interfaces become
designated ports and are typically located on an administrative boundary. If the bridge
receives superior STP BPDUs on a port that has root protection enabled, that port
transitions to a root-prevented STP state (inconsistency state) and the interface is
blocked. This blocking prevents a bridge that should not be the root bridge from being
elected the root bridge. After the bridge stops receiving superior STP BPDUs on the
interface with root protection, the interface returns to a listening state, followed by a
learning state, and ultimately back to a forwarding state. Recovery back to the forwarding
state is automatic.
When root protection is enabled on an interface, it is enabled for all the 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.
An interface can be configured for either root protection or loop protection, but not for
both.
Example: Configuring Root Protection to Enforce Root Bridge Placement in Spanning
Trees on EX Series Switches on page 1807
Example: Configuring Loop Protection to Prevent Interfaces from Transitioning from
Blocking to Forwarding in a Spanning Tree on EX Series Switches on page 1803
Example: Configuring BPDU Protection on STP Interfaces to Prevent STP
Miscalculations on EX Series Switches on page 1795
Example: Configuring BPDU Protection on non-STP Interfaces to Prevent STP
Miscalculations on EX Series Switches on page 1799
Understanding MSTP for EX Series Switches on page 1755
Understanding RSTP for EX Series Switches on page 1753
Understanding STP for EX Series Switches on page 1751
Understanding VSTP for EX Series Switches on page 1760
Chapter 70: Spanning-Tree Protocols—Overview
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