Protocols And Standards - 3Com MSR 50 Series Configuration Manual

Protocols and Standards

How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are
interconnected by a computed CST. Inside an MST region, multiple spanning trees
are generated through computing, each spanning tree called an MST instance.
Among these MST instances, instance 0 is the IST, while all the others are MSTIs.
Similar to STP, MSTP uses configuration BPDUs to compute spanning trees. The
only difference between the two protocols being in that what is carried in an
MSTP BPDU is the MSTP configuration on the device from which this BPDU is sent.
1 CIST computing
By comparison of configuration BPDUs, the device with the highest priority is
elected as the root bridge of the CIST. MSTP generates an IST within each MST
region through computing, and, at the same time, MSTP regards each MST region
as a single device and generates a CST among these MST regions through
computing. The CST and ISTs constitute the CIST of the entire network.
2 MSTI computing
Within an MST region, MSTP generates different MSTIs for different VLANs based
on the VLAN-to-instance mappings.
MSTP performs a separate computing process, which is similar to spanning tree
computing in STP, for each spanning tree. For details, refer to
on page 445.
In MSTP, a VLAN packet is forwarded along the following paths:
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.
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Implementation of MSTP 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 computing.
In addition to basic MSTP functions, many management-facilitating special
functions are provided, as follows:
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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Support for hot swapping of interface cards and active/standby switchover.
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MSTP is documented in:
IEEE 802.1D: Spanning Tree Protocol
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IEEE 802.1w: Rapid Spanning Tree Protocol
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IEEE 802.1s: Multiple Spanning Tree Protocol
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MSTP Overview 457
"How STP works"