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How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculated
CST. Inside an MST region, multiple spanning trees are calculated. Each spanning tree is an MSTI.
Among these MSTIs, MSTI 0 is the IST. Like STP, MSTP uses configuration BPDUs to calculate spanning
trees. An important difference is that an MSTP BPDU carries the MSTP configuration of the bridge from
which the BPDU is sent.
CIST calculation
The calculation of a CIST tree is also the process of configuration BPDU comparison. During this process,
the device with the highest priority is elected as the root bridge of the CIST. MSTP generates an IST within
each MST region through calculation. At the same time, MSTP regards each MST region as a single
device and generates a CST among these MST regions through calculation. The CST and ISTs constitute
the CIST of the entire network.
MSTI calculation
Within an MST region, MSTP generates different MSTIs for different VLANs based on the
VLAN-to-instance mappings. For each spanning tree, MSTP performs a separate calculation process
similar to spanning tree calculation in STP. For more information, see "
Calculation process of the STP
In MSTP, a VLAN packet is forwarded along the following paths:
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Within an MST region, the packet is forwarded along the corresponding MSTI.
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Between two MST regions, the packet is forwarded along the CST.
Implementation of MSTP on devices
MSTP is compatible with STP and RSTP. Devices that are running MSTP and that are used for spanning
tree calculation can identify STP and RSTP protocol packets.
In addition to basic MSTP functions, the following functions are provided for ease of management:
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Root bridge hold
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Root bridge backup
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Root guard
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BPDU guard
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Loop guard
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TC-BPDU guard
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BPDU drop.
Protocols and standards
The spanning tree protocols are documented in the following standards:
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IEEE 802.1d,
Media Access Control (MAC) Bridges
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IEEE 802.1w,
Part 3: Media Access Control (MAC) Bridges—Amendment 2: Rapid Reconfiguration
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IEEE 802.1s,
Virtual Bridged Local Area Networks—Amendment 3: Multiple Spanning Trees
