H3C Technologies H3C S5560 Series Switches - Manual (Page 107)

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spanning tree device are connected, so the device blocks one of the ports. The blocked port acts as
the backup.
• Edge port—Does not connect to any network device or network segment, but directly connects to a
user host.
• Master port—Acts as a port on the shortest path from the local MST region to the common root
bridge. The master port is not always located on the regional root. It is a root port on the IST or CIST
and still a master port on the other MSTIs.
• 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.
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. Table 9 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 9 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.