Understanding Mstp For Ex Series And Qfx Series Switches; Mstp Maps Multiple Vlans - Juniper EX9200 Features Manual

Spanning-Tree Protocols Feature Guide for EX9200 Switches

Understanding MSTP for EX Series and QFX Series Switches

MSTP Maps Multiple VLANs

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Ethernet networks are susceptible to broadcast storms if loops are introduced. However,
an Ethernet network needs to include loops because they provide redundant paths in
case of a link failure. Spanning-tree protocols address both of these issues because they
provide link redundancy while simultaneously preventing undesirable loops.
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. 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 MSTP.
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.
This topic describes:
MSTP Maps Multiple VLANs on page 78
Configuring MSTP Regions on page 79
Selecting a Spanning Tree Protocol on page 79
MSTP is an extension of RSTP that maps 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 might 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 reconvergence time of RSTP when a network, switch, or port failure occurs within
a spanning-tree instance.
MSTP creates a common and internal spanning tree (CIST) to interconnect and manage
all MSTP regions and even individual devices that run RSTP or STP, which are recognized
as distinct spanning-tree regions by MSTP. The CIST views each MSTP region as a virtual
bridge, regardless of the actual number of devices participating in the MSTP region, and
enables multiple spanning-tree instances (MSTIs) to link to other regions. The CIST is a
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