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

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Understanding MSTP for EX Series Switches

Copyright © 2010, Juniper Networks, Inc.
the receiving port on the nearest neighboring switch by using the proposal-agreement
handshake to ensure a loop-free topology.
RSTP also defines the concept of an edge port, which is a designated port that connects
to non-STP-capable devices, such as PCs, servers, routers, or hubs that are not connected
to other switches. Because edge ports connect directly to end stations, they cannot
create network loops and can transition to the forwarding state immediately, skipping
the listening and learning stages required by STP. You can manually configure edge ports,
and a switch can also detect edge ports by noting the absence of configuration BPDUs
from any attached systems. If an edge port receives a BPDU, it transitions to a regular
STP port.
By taking advantage of edge ports and point-to-point links, RSTP provides rapid
re-configuration of the spanning tree that can occur in less than one second. Contrasted
with the default 50-second re-convergence time based on STP timers (IEEE 802.1D),
RSTP provides critical support for networks carrying delay-sensitive traffic, such as voice
or video.
Understanding STP for EX Series Switches on page 1751
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
Example: Configuring Faster Convergence and Improving Network Stability with RSTP
on EX Series Switches on page 1761
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, but RSTP does not solve an inherent
problem in STP: All virtual local area networks (VLANs) within a local area network (LAN)
share the same spanning tree, which limits the number of forwarding paths for data
traffic. To address this problem, we recommend that you enable MSTP if you use VLANs,
unless your network requires the device compatibility provided by VSTP.
MSTP extends STP and RSTP functionality by mapping multiple independent
spanning-tree instances onto one physical topology. Each spanning-tree instance (STI)
includes one or more VLANs. Unlike in STP and RSTP configurations, a port may belong
to multiple VLANs and be dynamically blocked in one spanning-tree instance but
forwarding in another. This behavior significantly improves network resource utilization
by load-balancing across the network and maintaining switch CPU loads at moderate
levels. MSTP also leverages the fast re-convergence time of RSTP when a network,
switch, or port failure occurs within a spanning-tree instance.
When enabling MSTP, you define one or more MSTP regions. An MSTP region defines a
logical domain where MSTIs can be administered independently of MSTIs in other regions,
Chapter 70: Spanning-Tree Protocols—Overview
1755