3Com Switch 8800 Advanced Software V5 Configuration Guide

106 CHAPTER 11: MSTP CONFIGURATION
■ Port 3 and port 4 of device D connect downstream to other MST regions.
11 Port states
In MSTP, port states fall into the following tree:
■ Forwarding: the port learns MAC addresses and forwards user traffic;
■ Learning: the port learns MAC addresses but does not forwards user traffic;
■ Discarding: the port neither learns MAC addresses nor forwards user traffic.
n
When in different MST instances, a port can be in different states.
A port state is not exclusively associated with a port role. Table 13 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).
How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are
interconnected by a computed CST. Inside an MST region, multiple spanning trees
are generated through computing, each spanning tree called an MST instance.
Among these MST instances, instance 0 is the IST, while all the others are MSTIs.
Similar to STP, MSTP uses configuration BPDUs to compute spanning trees. The
only difference between the two protocols being in that what is carried in an
MSTP BPDU is the MSTP configuration on the device from which this BPDU is sent.
1 CIST computing
By comparison of configuration BPDUs, one device with the highest priority is
elected as the root bridge of the CIST. MSTP generates an IST within each MST
region through computing, and, at the same time, MSTP regards each MST region
as a single device and generates a CST among these MST regions through
computing. The CST and ISTs constitute the CIST of the entire network.
2 MSTI computing
Within an MST region, MSTP generates different MSTIs for different VLANs based
on the VLAN-to-instance mappings.
MSTP performs a separate computing process, which is similar to spanning tree
computing in STP, for each spanning tree. For details, refer to
“How STP works”
on page 95.
In MSTP, a VLAN packet is forwarded along the following paths:
■ Within an MST region, the packet is forwarded along the corresponding MSTI.
Tabl e 13 Ports states supported by different port roles
Role\State
Root
port/Master
port
Designated
port
Alternate port Backup port
Forwarding √ √ - -
Learning √ √ - -
Discarding √ √ √ √