Owner's Manual

546 | Chapter 18. STP
NETGEAR 8800 User Manual
Port Roles
RSTP uses information from BPDUs to assign port roles for each LAN segment. Port roles
are not user-configurable. Port role assignments are determined based on the following
criteria:
• A unique bridge identifier (MAC address) associated with each bridge
• The path cost associated with each bridge port
• A port identifier associated with each bridge port
RSTP assigns one of the following port roles to bridge ports in the network, as described in
Table 52.
When RSTP stabilizes, all:
• Root ports and designated ports are in the forwarding state.
• Alternate ports and backup ports are in the blocking state.
RSTP makes the distinction between the alternate and backup port roles to describe the
rapid transition of the alternate port to the forwarding state if the root port fails.
Link Types
With RSTP, you can configure the link type of a port in an STPD. RSTP tries to rapidly move
designated point-to-point links into the forwarding state when a network topology change or
failure occurs. For rapid convergence to occur, the port must be configured as a point-to-point
link.
Table 53 describes the link types.
Table 52. RSTP Port Roles
Port Role Description
Root Provides the shortest (lowest) path cost to the root bridge. Each bridge has only one root port; the
root bridge does not have a root port. If a bridge has two or more ports with the same path cost,
the port with the best port identifier (lowest MAC address) becomes the root port.
Designated Provides the shortest path connection to the root bridge for the attached LAN segment. To prevent
loops in the network, there is only one designated port on each LAN segment. To select the
designated port, all bridges that are connected to a particular segment listen to each other’s
BPDUs and agree on the bridge sending the best BPDU. The corresponding port on that bridge
becomes the designated port. If there are two or more ports connected to the LAN, the port with
the best port identifier becomes the designated port.
Alternate Provides an alternate path to the root bridge and the root port.
Backup Supports the designated port on the same attached LAN segment. Backup ports exist only when
the bridge is connected as a self-loop or to a shared-media segment.
Disabled A port in the disabled state does not participate in RSTP; however, it will forward traffic and learn
new MAC source addresses.