76
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
During the CIST calculation, the following process takes place:
•
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.
•
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 "
."
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. 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:
•
Root bridge hold
•
Root bridge backup
•
Root guard
•
BPDU guard
•
Loop guard
•
TC-BPDU guard
•
Port role restriction
•
TC-BPDU transmission restriction
Protocols and standards
MSTP is documented in the following 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
•
IEEE 802.1Q-REV/D1.3,
Media Access Control (MAC) Bridges and Virtual Bridged Local Area
Networks —Clause 13: Spanning tree Protocols
Spanning tree configuration task lists
Before configuring a spanning tree, complete the following tasks:
