RuggedCom RS400 manual Bridge and Port Roles

Spanning Tree

The learning state is entered when the port is preparing to play an active part in the network. The port learns addresses in this state but does not participate in frame transfer. In a network of RSTP bridges the time spent in this state is usually quite short. RSTP bridges operating in STP compatibility mode will spend 6 to 40 seconds in this state.

After “learning” the bridge will place the port in the forwarding state. The port both learns addresses and participates in frame transfer while in this state.

Note: ROS™ introduces two more states, Disabled and Link Down. Introduced purely for purposes of management, these states may be considered sub-classes 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 designated if it is the best port to service the LAN segment it is connected to. All bridges on the same LAN segment listen to each others’ messages and agree on who is the designated bridge. The ports of other bridges on the segment must become either root, alternate or backup ports.

A

DP

Root

3Bridge

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DP DP

RP = Root Port

DP = Designated Port

AP = Alternate Port

BP = Backup Port

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Bridge

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D

RP

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Bridge

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Figure 95: 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 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

RS400 135 ROS™ v3.5