92
CIST. However, 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, 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.
lists the port states that each 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 10 Port states that different port roles support
Port role (right)
Port state (below)
Root
port/master
port
Designated
port
Alternate port
Backup port
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, called MSTIs, are calculated. 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
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 frame is forwarded along the following paths:
