3Com S7902E, Configuration Manual

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1-14
Port states
In MSTP, port states fall into the following three:
z
Forwarding: the port learns MAC addresses and forwards user traffic;
z
Learning: the port learns MAC addresses but does not forward user traffic;
z
Discarding: the port neither learns MAC addresses nor forwards user traffic.
When in different MSTIs, a port can be in different states.
A port state is not exclusively associated with a port role. Table 1-6 lists the port state(s) supported by
each port role (“ √ ” indicates that the port supports this state, while “—“ indicates that the port does not
support this state).
Table 1-6
Port states supported by different port roles
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 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 the IST, while all the others are MSTIs. Similar to STP, 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-instance mappings. MSTP performs a separate calculation process, which is similar to
spanning tree calculation in STP, for each spanning tree. For details, refer to
How STP works .
In MSTP, a VLAN packet is forwarded along the following paths: