28. Spanning Tree
ROX™ v2.2 User Guide
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RuggedBackbone™ RX1500
ROX™ introduces two more states - Disabled and Link Down. Introduced purely for
purposes of management, these states may be considered subclasses of the RSTP
Discarding state. The Disabled state refers to links for which RSTP has been disabled. The
Link Down state refers to links for which RSTP is enabled but are currently down.
Role
There are four RSTP port roles: Root, Designated, Alternate and Backup.
If the bridge is not the root bridge, it must have a single Root Port. The Root Port is the “best” (i.e.
quickest) way to send traffic to the root bridge.
A port is marked as Designated if it is the best port to serve the LAN segment it is connected to. All
bridges on the same LAN segment listen to each others’ messages and agree on which bridge is the
Designated Bridge. The ports of other bridges on the segment must become either Root, Alternate or
Backup ports.
Figure 28.2. Bridge and Port Roles
A port is alternate when it receives a better message from another bridge on the LAN segment it is
connected to. The message that an Alternate Port receives is better than the port itself would generate,
but not good enough to convince it to become the Root Port. The port becomes the alternate to the
current Root Port and will become the new Root Port should the current Root Port fail. The Alternate
Port does not participate in the network.
A port is a Backup Port when it receives a better message from the LAN segment it is connected to,
originating from another port on the same bridge. The port is a backup for another port on the bridge
and will become active if that port fails. The Backup Port does not participate in the network.
