Spanning Tree Protocols Help Prevent Broadcast Storms; Rstp Is An Enhancement Of The Original Stp; Port Roles Determine Participation In The Spanning Tree - Juniper EX9200 Features Manual

Spanning-Tree Protocols Feature Guide for EX9200 Switches

Spanning Tree Protocols Help Prevent Broadcast Storms

RSTP is an Enhancement of the Original STP

Port Roles Determine Participation in the Spanning Tree

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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 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.
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
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