How Mstp Works - HP 1910 User Manual

Forwarding—The port receives and sends BPDUs, learns MAC addresses, and forwards user
•
traffic.
Learning—The port receives and sends BPDUs, learns MAC addresses, but does not forward user
•
traffic. Learning is an intermediate port state.
Discarding—The port receives and sends BPDUs, but does not learn 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 59 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 interconnected by a
calculated CST. Inside an MST region, multiple spanning trees are calculated, each being an MSTI
(Among these MSTIs, MSTI 0 is called the CIST). Similar to RSTP, 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.
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, 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.
MSTI calculation
Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to-MSTI
mappings. MSTP performs a separate calculation process, which is similar to spanning tree calculation
in STP/RSTP, for each spanning tree. 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
√ √
√ √
√ √
155
Table 59 lists the port states that each port role
Alternate port
—
—
√
"How STP works."
Backup port
—
—
√