H3C Technologies H3C SecPath F1000-E - Manual (Page 206)

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In Figure 5 , devices A, B, C, and D constitute an MST region. Port 1 and port 2 of device A are connected
to the common root bridge, port 5 and port 6 of device C form a loop, port 3 and port 4 of device D are
connected downstream to the other MST regions.
Port states
In MSTP, port states fall into the following three:
• Forwarding: the port learns MAC addresses and forwards user traffic;
• Learning: the port learns MAC addresses but does not forward user traffic;
• Discarding: the port does not learn MAC addresses or forwards user traffic.
NOTE:
A port can have different port states in different MSTIs.
A port state is not exclusively associated with a port role. Table 6 lists the port state(s) supported by each
port role. (A check (√) indicates that the port state is available for the corresponding port role; an em
dash (—) indicates that the port state is not available for the corresponding port role.)
Table 6 Ports states supported by different port roles
Port role
Port state
Root
port/master
port
Designated
port
Boundary port Alternate port Backup port
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 called 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 details, refer to How STP Works .