3Com Switch 4200G Configuration Guide

MSTP Overview 115
IST
An internal spanning tree (IST) is a spanning tree in an MST region.
ISTs together with the common spanning tree (CST) form the common and internal
spanning tree (CIST) of the entire switched network. An IST is a special MSTI; it
belongs to an MST region and is a branch of CIST. In Figure 36, each MST region has
an IST, which is a branch of the CIST.
CST
A CST is the spanning tree in a switched network that connects all MST regions in the
network. If you regard each MST region in the network as a switch, then the CST is
the spanning tree generated by STP or RSTP running on the “switches”. In Figure 36,
the lines in red depict the CST.
CIST
A CIST is the spanning tree in a switched network that connects all switches in the
network. It comprises the ISTs and the CST. In Figure 36, the ISTs in the MST regions
and the CST connecting the MST regions form the CIST.
Region root
A region root is the root of the IST or an MSTI in a MST region. Different spanning
trees in an MST region may have different topologies and thus have different region
roots. In region D0 shown in Figure 36, the region root of MSTI 1 is switch B, and the
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 36 is a switch in region A0.
Region edge port
A region edge port is located on the edge of an MST region and is used to connect
the MST region to another MST region, a STP-enabled region, or an RSTP-enabled
region.
Port roles
In MSTP, the following port roles exist: root port, designated port, master port, region
edge port, alternate port, and backup port.
■ A root port is used to forward packets to the root.
■ A designated port is used to forward packets to a downstream network segment
or switch.
■ A master port connects a 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.
■ An alternate port can be a backup port of a master or root port. When it operates
as a backup port of a master port, it becomes the master port if the existing
master port is blocked.
■ A loop occurs when two ports of a switch are connected to each other. In this
case, the switch blocks one of the two ports. The blocked port is a backup port.