3com 4210 PWR 9-Port 3CR17341-91-ME User Manual
Product codes
3CR17341-91-ME
150
C
HAPTER
14: MSTP C
ONFIGURATION
Region root
A region root is the root of the IST or an MSTI in an MST region. Different
spanning trees in an MST region may have different topologies and thus have
different region roots.
spanning trees in an MST region may have different topologies and thus have
different region roots.
In region D0 shown in Figure 49, the region root of MSTI 1 is switch B, and the
region root of MSTI 2 is switch C.
region root of MSTI 2 is switch C.
Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the
network shown in Figure 49 is a switch in region A0.
network shown in Figure 49 is a switch in region A0.
Port role
During MSTP calculation, the following port roles exist: root port, designated port,
master port, region edge port, alternate port, and backup port.
master port, region edge port, alternate port, and backup port.
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A root port is used to forward packets to the root.
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A designated port is used to forward packets to a downstream network
segment or switch.
segment or switch.
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A master port connects an MST region to the common root. The path from the
master port to the common root is the shortest path between the MST region
and the common root.
master port to the common root is the shortest path between the MST region
and the common root.
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A region edge port is located on the edge of an MST region and is used to
connect one MST region to another MST region, an STP-enabled region or an
RSTP-enabled region
connect one MST region to another MST region, an STP-enabled region or an
RSTP-enabled region
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An alternate port is a secondary port of a root port or master port and is used
for rapid transition. With the root port or master port being blocked, the
alternate port becomes the new root port or master port.
for rapid transition. With the root port or master port being blocked, the
alternate port becomes the new root port or master port.
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A backup port is the secondary port of a designated port and is used for rapid
transition. With the designated port being blocked, the backup port becomes
the new designated port fast and begins to forward data seamlessly. When two
ports of an MSTP-enabled switch are interconnected, the switch blocks one of
the two ports to eliminate the loop that occurs. The blocked port is the backup
port.
transition. With the designated port being blocked, the backup port becomes
the new designated port fast and begins to forward data seamlessly. When two
ports of an MSTP-enabled switch are interconnected, the switch blocks one of
the two ports to eliminate the loop that occurs. The blocked port is the backup
port.
In Figure 50, switch A, switch B, switch C, and switch D form an MST region. Port
1 and port 2 on switch A connect upstream to the common root. Port 5 and port
6 on switch C form a loop. Port 3 and port 4 on switch D connect downstream to
other MST regions. This figure shows the roles these ports play.
1 and port 2 on switch A connect upstream to the common root. Port 5 and port
6 on switch C form a loop. Port 3 and port 4 on switch D connect downstream to
other MST regions. This figure shows the roles these ports play.
n
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A port can play different roles in different MSTIs.
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The role a region edge port plays is consistent with the role it plays in the CIST.
For example, port 1 on switch A in Figure 50 is a region edge port, and it is a
master port in the CIST. So it is a master port in all MSTIs in the region.
For example, port 1 on switch A in Figure 50 is a region edge port, and it is a
master port in the CIST. So it is a master port in all MSTIs in the region.