How Mstp Works - HP 6125G Configuration Manual

Boundary port—Connects an MST region to another MST region or to an STP/RSTP-running device.
•
In MSTP calculation, a boundary port's role on an MSTI is consistent with its role on the CIST. But
that is not true with master ports. A master port on MSTIs is a root port on the CIST.
Port states
In MSTP, a port can be in one of the following states:
Forwarding—The port receives and sends BPDUs, obtains MAC addresses, and forwards user
•
traffic.
• Learning—The port receives and sends BPDUs, obtains MAC addresses, but does not forward user
traffic. Learning is an intermediate port state.
Discarding—The port receives and sends BPDUs, but does not obtain MAC addresses or forward
•
user traffic.
When in different MSTIs, a port can be in different states. A port state is not exclusively associated with
a port role. Table 12
port supports this state, while a dash [—] indicates that the port does not support this state.)
Table 12 Port states that different port roles support
Port role (right)
Port state
(below)
Forwarding
Learning
Discarding

How MSTP works

MSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculated
CST. Inside an MST region, multiple spanning trees are calculated. Each spanning tree is an MSTI.
Among these MSTIs, MSTI 0 is the IST. 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 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. 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-instance mappings. For each spanning tree, MSTP performs a separate calculation process
similar to spanning tree calculation in STP. For more information, see
algorithm."
In MSTP, a VLAN packet is forwarded along the following paths:
• Within an MST region, the packet is forwarded along the corresponding MSTI.
lists the port states that each port role supports. (A check mark [√] indicates that the
Root port/master
Designated port
port
√ √
√ √
√ √
Alternate port
—
—
√
61
Backup port
—
—
√
"Calculation process of the STP