Changing Port States - Nokia ESB26 User Manual

13.
•
a Backup Port - if the Bridge is the Designated Bridge for the attached LAN
OR OTHERWISE
• an Alternate Port.
Alternate and Backup Ports
An Alternate Port offers a path in the direction of the Root Bridge alternate to that provided
by the Bridge's Root Port.
A Backup Port acts as a backup for the path provided by a Designated Port in the direction of
the leaves of the Spanning Tree. Backup Ports exist only where a given Bridge has two or
more connections to a given LAN, Therefore, backup ports (and the Designated Ports that
they back up) can exist only where two ports are connected together in loopback by a point to
point link, or where the Bridge has two or more connections to a shared media LAN segment.
The distinction between the Alternate and Backup Port Roles was introduced in RSTP in
order to describe the possibility of the rapid transition of an Alternate port to forwarding if the
Root Port fails.
Point-To-Point Links
Some of the rapid state transitions that are possible within RSTP depend on whether the Port
concerned can be connected to only one other Bridge (i.e., it is served by a point-to-point
LAN segment), or to two or more Bridges (i.e., it is served by a shared medium LAN
segment).
Rapid transition of a Designated Port to Forwarding is possible only if the LAN segment
associated with the Port is point-to-point, or if the port is defined to be an edge Port.
Otherwise, the transition of a Designated Port from Discarding to Learning and from learning
to Forwarding occurs with a delay of Forward Delay.

Changing Port States

The Port States are controlled by a state machine, designed to maximize connectivity without
introducing temporary data loops in the network. The state machine attempts to transition
Root Ports and Designated Ports to the Forwarding Port State, and Alternate Ports and
Backup Ports to the Discarding Port State, as rapidly as possible.
Transitions to the Discarding Port State can be simply effected without the risk of data loops.
Transition of a Port to the Forwarding Port State needs to be consistent with the Port Roles
assigned to other Ports in the region of the network.
A Bridge knows that the transition to the Forwarding Port State can be made if:
1. The Port Role has been Root Port or Designated Port long enough FOR:
spanning Tree information supporting this role assignment to have reached all Bridges in
the network,
AND FOR
MN700004 Rev 01
Rapid Spanning Tree Protocol (RSTP)
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