Fortinet Version 3.2.0 Manual

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Spanning Tree Protocol

Spanning tree protocol is a link-management protocol that ensures a loop-free Layer 2 network topology.

FortiSwitch supports the Multiple Spanning Tree Protocol (MSTP), which is defined in the IEEE 802.1Q standard.

MSTP Overview and terminology

MSTPsupports multiple spanning tree instances, where each instance carries traffic for one or more VLANs (the

mapping of VLANs to instances is configurable).

MSTP is backward-compatible with STPand RSTP. A given Layer 2 network may contain switches that are

running MSTP, STPor RSTP.

MSTPis built on RSTP, so it provides fast recovery from network faults and fast convergence times.

Regions

A region is a set of interconnected switches that have the same MSTconfiguration (region name, MSTrevision

number and VLAN-to-instance mapping). A network may have any number of regions. Regions are independent

of each other (VLAN-to-instance mapping is different in each region).

FortiSwitch supports 15 MST instances in a region. Multiple VLANs can be mapped to each MSTinstance. Each

switch in the region must have the identical mapping of VLANs to instances.

The MSTregion acts like a single bridge to adjacent MSTregions and to non-MST STPprotocols.

IST

Instance 0 is a special instance, called the IST. ISTis a spanning tree that connects all of the MST switches in a

region. All VLANs are assigned to the IST.

ISTis the only instance that exchanges BPDUs. The MSTPBPDUcontains information for each MSTPinstance

(captured in an M-record). The M-records are added to the end of a regular RSTPBPDU. This allows MSTP

region to inter-operate with an RSTPswitch.

CST

The Common Spanning Tree (CST)interconnects the MSTregions and all instances of STP or RSTP that are

running in the network.

Hop Count and Message Age

MST does not use the BPDUmessage age within a region. The message-age and maximum-age fields in the

BPDUare propagated unchanged within the region.

FortiSwitchOS-3.2.0