MSTP Overview 211
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 generated through calculation, 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 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.
1 CIST calculation
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 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.
2 MSTI calculation
Within an MST region, MSTP generates different spanning tree instances for
different VLANs based on the VLAN-to-instance mappings. MSTP performs a
separate calculation process, which is similar to spanning tree calculation in STP,
for each spanning tree. For details, refer to “How STP works” on page 199.
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.
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 calculation.
In addition to basic MSTP functions, many management-facilitating special
functions are provided, as follows:
■ Root bridge hold
■ Root bridge backup
■ Root guard
■ BPDU guard
■ Loop guard
■ TC-BPDU guard
Protocols and Standards MSTP is documented in:
■ IEEE 802.1d: Spanning Tree Protocol
Table 42 Ports states supported by different port roles
Role/
State
Root port/
Master port
Designated port Alternate port Backup port
Forwarding ‚àö ‚àö - -
Learning ‚àö ‚àö - -
Discarding ‚àö ‚àö ‚àö ‚àö
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