HP VPN Firewall Appliances Network Management Configuration Guide

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MST region
A multiple spanning tree region (MST region) consists of multiple devices in a switched network and the
network segments among them. All these devices have the following characteristics:
• A spanning tree protocol is enabled.
• Same region name.
• Same VLAN-to-instance mapping configuration.
• Same MSTP revision level.
• Physically linked together.
Multiple MST regions can exist in a switched network. You can assign multiple devices to the same MST
region. In Figure 42,
the switched network comprises MST region 1 through MST region 4, and all
devices in e
ach MST region have the same MST region configuration.
MSTI
MSTP can generate multiple independent spanning trees in an MST region, and each spanning tree is
mapped to specific VLANs. Each spanning tree is called a "multiple spanning tree instance (MSTI)."
In Figure 43, M
ST region 3 compri
ses MSTI 1, MSTI 2, and MSTI 0.
VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes mapping relationships
between VLANs and MSTIs.
In Figure 43, the VL
AN-to-instance mapping table o
f MST region 3 is: VLAN 1 to MSTI 1, VLAN 2 and
VLAN 3 to MSTI 2, and other VLANs to MSTI 0. 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 blue lines in Figure 42 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 42,
MSTI 0 is the IST in MST region 3.
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 42, t
he ISTs (MSTI 0) in all MST regions plus the inter-region CST constitute the CIST of the entire
network.
Regional root
The root bridge of the IST or an MSTI within an MST region is the regional root of the IST or MSTI. Based
on the topology, different spanning trees in an MST region might have different regional roots.
In MST region 3 in Figure 43,
the regional root of MSTI 1 is Device B, the regional root of MSTI 2 is
Device C, and the regional ro
ot of MSTI 0 (also known as the IST) is Device A.