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 the following table.

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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.

When RSTP stabilizes:

  • All root ports and designated ports are in the forwarding state.

  • All 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.

To prevent a port from becoming an alternate or backup port, use the command:

configure stpd stpd_name ports active-role enable port .

To revert to the default that allows a port to be elected to any STP port role, use the command:

configure stpd stpd_name ports active-role disable port

To view the active-role status, use the command: show stpd ports.