H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 77)

66
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 comparing configuration BPDUs. 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 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 algorithm ."
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. Devices that run MSTP can recognize STP and RSTP protocol
packets used for spanning tree calculation.
In addition to basic MSTP functions, the following functions are provided for ease of management:
• Root bridge hold.
• Root bridge backup.
• Root guard.
• BPDU guard.
• Loop guard.
• TC-BPDU guard.
• BPDU drop.
Protocols and standards
• IEEE 802.1d, Media Access Control (MAC) Bridges
• IEEE 802.1w, Part 3: Media Access Control (MAC) Bridges—Amendment 2: Rapid Reconfiguration
• IEEE 802.1s, Virtual Bridged Local Area Networks—Amendment 3: Multiple Spanning Trees
Spanning tree configuration task lists
Before configuring a spanning tree, you must determine the spanning tree protocol to be used (STP, RSTP,
PVST, or MSTP) and plan device roles (the root bridge or leaf node).