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How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are interconnected by a
calculated CST. Inside an MST region, multiple spanning trees are calculated, each being an MSTI.
Among these MSTIs, MSTI 0 is called the CIST. Similar to RSTP, MSTP uses configuration BPDUs to
calculate spanning trees. The only difference between the two protocols is that an MSTP BPDU carries the
MSTP configuration on the device from which this 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, and, 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-MSTI
mappings. MSTP performs a separate calculation process, which is similar to spanning tree calculation
in STP/RSTP, for each spanning tree.
In MSTP, a VLAN packet is forwarded along the following paths:
• Within an MST region, the packet is forwarded along the corresponding MSTI.
• Between two MST regions, the packet is forwarded along the CST.
MSTP implementation on devices
MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be recognized by devices
running MSTP and used for spanning tree calculation.
In addition to basic MSTP functions, the device provides the following functions for ease of management:
• Root bridge hold
• Root bridge backup
• Root guard
• BPDU guard
• Loop guard
• TC-BPDU (a message that notifies the device of topology changes) guard
• Support for the hot swapping of interface boards and switchover of the active and standby main
boards.
Protocols and standards
• IEEE 802.1d, Spanning Tree Protocol
• IEEE 802.1w, Rapid Spanning Tree Protocol
• IEEE 802.1s, Multiple Spanning Tree Protocol
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