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

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Understanding RSTP for EX Series Switches

Copyright © 2010, Juniper Networks, Inc.
from the learning to the forwarding state, thereby allowing frames to be received and
forwarded by the port.
For STP to recover from a link failure, therefore, it takes approximately 50 seconds: 20
seconds for a BPDU to age out, 15 seconds for the listening state, and 15 seconds for the
learning state. This recalculation of the spanning tree is a time-consuming process and
can result in delayed message delivery as ports transition between states. Users perceive
these delays as service interruptions and certain applications, protocols, or processes
can time out. These results are unacceptable in current high-availability networks, which
led to the evolution of STP to RSTP.
Configuring STP (CLI Procedure) on page 1814
Understanding RSTP for EX Series Switches on page 1753
Understanding MSTP for EX Series Switches on page 1755
Understanding VSTP for EX Series Switches on page 1760
Understanding Layer 2 Protocol Tunneling on EX Series Switches on page 1520
Juniper Networks EX Series Ethernet 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). The default factory
configuration for EX Series switches uses RSTP. If your network includes 802.1D 1998
bridges, you can explicitly configure STP. Note that when doing so, the EX Series switches
use the IEEE 802.1D 2004 specification, force version 0, which is an RSTP configuration
that is compatible with basic STP. If you use VLANs, we recommend that you enable
MSTP unless your network requires the device compatibility provided by VSTP.
RSTP is an evolution of the STP IEEE 802.1D protocol designed to provide faster spanning
tree re-convergence after a switch port, switch, or LAN failure. STP takes up to 50 seconds
to respond to topology changes while RSTP responds to changes within the timeframe
of three hello BPDUs (Bridge Protocol Data Units), or 6 seconds. RSTP calculates the
spanning tree in the same manner as STP, but re-convergence after a connectivity failure
works differently. The key difference between STP and RSTP is that the latter does not
use timers (rather a handshake) to transition between port states and roles.
A port's state determines how it processes a frame. A port's role determines how it
participates in the spanning tree. STP places each port of a designated bridge in one of
five states and assigns it a role as root, designated, or non-designated port. (See
"Understanding STP for EX Series Switches" on page 1751.) RSTP assigns a port to one of
three states, simplifying the process for a port to enter the forwarding state, and
establishes new port roles that serve as back-ups for a failed root or designated port on
a designated bridge.
The three port states used by RSTP are:
Chapter 70: Spanning-Tree Protocols—Overview
1753