HP 1920 Gigabit Ethernet Switch Series User Manual

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VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping
relationships between VLANs and MSTIs.
In Figure 176,
the VLAN-to-instance mapping table of region A0 is: VLAN 1 is mapped to MSTI 1, VLAN
2 to MSTI 2, and the rest to CIST. MSTP achieves load balancing by means of the VLAN-to-instance
mapping table.
CST
The common spanning tree (CST) is a single spanning tree that connects all MST regions in a switched
network. If you regard each MST region as a device, the CST is a spanning tree calculated by these
devices through STP or RSTP.
The red lines in Figure 176 r
epresent the CST.
IST
An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is also called MSTI 0, a
special MSTI to which all VLANs are mapped by default.
In Figure 176,
the CIST has a section in each MST region, and this section is the IST in the respective MST
region.
CIST
The common and internal spanning tree (CIST) is a single spanning tree that connects all devices in a
switched network. It consists of the ISTs in all MST regions and the CST.
In Figure 176, the I
STs in all MST regions plus the inter-region CST constitute the CIST of the entire
network.
Regional root bridge
The root bridge of the IST or an MSTI within an MST region is the regional root bridge of the IST or the
MSTI. Based on the topology, different spanning trees in an MST region might have different regional
roots.
As shown in Figure 176,
the regional root of MSTI 1 in region D0 is device B, and that of MSTI 2 is device
C.
Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 176, f
or example, the common root bridge is a device in region A0.
Port roles
A port can play different roles in different MSTIs. As shown in Figure 177, an MST region has device A,
device B, device C, and device D. 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, and port 3 and port 4 of device D are connected
downstream to the other MST regions.

HP 1920 Gigabit Ethernet Switch Series Specifications

General

Brand	HP
Model	1920 Gigabit Ethernet Switch Series
Category	Switch
Language	English