How Mstp Works - HP A5120 EI Series Configuration Manual

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.
NOTE:
When in different MSTIs, a port can be in different states.
A port state is not exclusively associated with a port role.
supports. (A check mark [√] indicates that the 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 on 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, which is
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.

Root port/master
Designated port
port
√ √
√ √
√ √
Table 12 lists the port states that each port role
Alternate port
—
—
√
―How STP
66
Backup port
—
—
√
works.‖