HP VPN Firewall Appliances Network Management Configuration Guide

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Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 42, the common root bridge is a devi
ce in MST region 1.
Port roles
A port can play different roles in different MSTIs. As shown in Figure 44, an MST region comprises
Device A, Device B, Device C, and Device D. Port A1 and port A2 of Device A connect to the common
root bridge. Port B2 and Port B3 of Device B form a loop. Port C3 and Port C4 of Device C connect to
other MST regions. Port D3 of Device D directly connects to a host.
Figure 44 Port roles
MSTP calculation involves the following port roles:
• Root port—Forwards data for a non-root bridge to the root bridge. The root bridge does not have
any root port.
• Designated port—Forwards data to the downstream network segment or device.
• Alternate port—The backup port for a root port or master port. When the root port or master port
is blocked, the alternate port takes over.
• Backup port—The backup port of a designated port. When the designated port is invalid, the
backup port becomes the new designated port. A loop occurs when two ports of the same
spanning tree device are interconnected, so the device blocks one of the ports. The blocked port
acts as the backup.
• Edge port—An edge port does not connect to any network device or network segment, but directly
connects to a user host.
• Master port—A port on the shortest path from the local MST region to the common root bridge. The
master port is not always located on the regional root. It is a root port on the IST or CIST and still a
master port on other MSTIs.