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Understanding MSTP for EX Series Switches

Ethernet networks are susceptible to broadcast storms if loops are introduced. However, an Ethernet network should always 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 such as Configuring BPDU Protection on Edge Interfaces to Prevent STP Miscalculations on EX Series Switches 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 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). This topic explains MSTP.

Note: If you are using Junos OS for EX 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.

MSTP Maps Multiple VLANs

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 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.

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 MSTIs to link to other regions. The CIST is a single topology that connects all switches (STP, RSTP, and MSTP devices) through an active topology, ensuring connectivity between LANs and devices within a bridged network. This functionality provided by MSTP enables you to better utilize network resources while remaining backward-compatible with older network devices.

Configuring MSTP Regions

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, setting the boundary for Bridge Protocol Data Units (BPDUs) sent by one MSTI. An MSTP region is a group of switches that is defined by three parameters:

• Region name—User-defined alphanumeric name for the region.
• Revision level—User-defined value that identifies the region.
• Mapping table—Numerical digest of VLAN-to-instance mappings.

An MSTP region can support up to 64 MST instances, and each MSTI can support from 1 to 4094 VLANs. When you define a region, MSTP automatically creates an internal spanning-tree instance (IST instance 0) that provides the root switch for the region and includes all currently configured VLANs that are not specifically assigned to a user-defined Multiple Spanning-Tree Instance (MSTI). An MSTI includes all static VLANs that you specifically add to it. The switch places any dynamically created VLANs in the IST instance by default, unless you explicitly map them to another MSTI. Once you assign a VLAN to a user-defined MSTI, the switch removes the VLAN from the IST instance.

Selecting a Spanning-Tree Protocol

The default factory configuration for EX Series switches is RSTP, a faster version of STP. To determine which spanning-tree protocol is best for your situation, see Table 1 below.