RuggedCom RS400 manual Edge Ports, Point-to-Point and Multipoint Links, Path and Port Costs

Spanning Tree

the bridge and will become active if that port fails. The backup port does not participate in the network.

5.1.2 Edge Ports

A port may be designated an edge port if it is directly connected to an end station. As such, it cannot create bridging loops in the network and can thus directly transition to forwarding, skipping the listening and learning stages.

Edge ports that receive configuration messages immediately lose their edge port status and become normal spanning tree ports. A loop created on an improperly connected edge port is thus quickly repaired.

Because an edge port services only end stations, topology change messages are not generated when its link toggles.

5.1.3 Point-to-Point and Multipoint Links

RSTP uses a peer-peer protocol called Proposing-Agreeing to ensure transitioning in the event of a link failure. This protocol is point-to-point and breaks down in multipoint situations, i.e. when more than two bridges operate on a shared media link.

If RSTP detects this circumstance (based upon the port’s half duplex state after link up) it will switch off Proposing-Agreeing. The port must transition through the learning and forwarding states, spending one forward delay in each state.

There are circumstances in which RSTP will make an incorrect decision about the point-to-point state of the link simply by examining the half duplex status, namely:

•The port attaches only to a single partner, but through a half duplex link.

•The port attaches to a shared media hub through a full duplex link. The shared media link attaches to more than one RSTP enabled bridge.

In such cases the user may configure the bridge to override the half duplex determination mechanism and force the link to be treated in the proper fashion.

5.1.4 Path and Port Costs

The STP path cost is the main metric by which root and designated ports are chosen2. The path cost for a designated bridge is the sum of the individual port costs of the links between the root bridge and that designated bridge. The port with the lowest path cost is the best route to the root bridge and is chosen as the root port.

How Port Costs Are Generated

Port costs can be generated either as a result of link auto-negotiation or manual configuration.

When the link auto-negotiation method is used, the port cost is derived from the speed of the link. This method is useful when a well-connected network has been established. It can be used

2In actuality the primary determinant for root port selection is the root bridge ID. Bridge ID is important mainly at network startup when the bridge with the lowest ID is elected as the root bridge. After startup (when all bridges agree on the root bridge’s ID) the path cost is used to select root ports. If the path costs of candidates for the root port are the same, the ID of the peer bridge is used to select the port. Finally, if candidate root ports have the same path cost and peer bridge ID, the port ID of the peer bridge is used to select the root port. In all cases the lower ID, path cost or port ID is selected as the best.

ROS™ v3.5 136 RS400