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What is a purpose of Port Priority in the RSTP and how can we access Port priority in the BMENOC3xx modules?

To access the Port Priority in the eNOC modules, we have to use the eNOC DTM in the Advanced Mode. The Port priority configuration is available for the Port2 and Port3 of the eNOC module because Port 2 and 3 support the RSTP.

In the RSTP, we have multiple nodes. One of the nodes becomes the Root bridge and the rest of them act as Participants.

We have three types of ports in the network:
Root Port (RP): This is the port that leads towards the Root Bridge.
Designated Port (DP): This is the port that leads away from the root bridge (Port Forwarding).
Blocking Port (BLK): Non-Designated port where it's blocking.

All the ports on a root bridge are the designated ports (DP)/Port Forwarding.

Note: Don’t mix up the Root bridge and Root ports both are not the same. The root port is existing on the Participants.

To avoid the loop in the network, one of the links must be in the blocked state.

When the system boots up, they exchange the BPDUs and the first thing they do is to elect the Root Bridge. The lower Priority will become the Root Bridge.

In the eNOC, we can select Root Bridge priority settings as mentioned in the picture below.

For example, Modicon Switch is acting as a Root Bridge/Root switch, and both ports become DP. Both BMENOC0301modules are acting as participants 1 and 2. Both BMENOC0301 got the RP leading towards the Root bridge but in between them one is DP and one is blocking (BLK) this selection is done by the port priority in the RSTP process. As shown in the picture below.

When a loop occurs in network topology, the spanning tree can use the port priority value for the ports to decide which port must be put in a forwarding state. The port priority is only used to determine the topology if the loop in the network cannot be resolved using bridge IDs or path cost.

If a higher priority (lower numerical value) is assigned to a port, STP uses forwarding first. When a lower priority (higher numerical value) is assigned to a port, STP uses forwarding last. If all ports have the same priority values, the spanning tree puts the lowest numbered interface in the forwarding state and blocks all other interfaces.

It requires network expertise to configure the Port Priority these changes must be done with care after understanding the overall network and what would be the impact of the port priority change on the network.

The below picture is taken from the Control Expert help file and explains the difference between the RP and DP ports.

To summarize, the port priority is used in the RSTP process to decide which port will be blocking. Its helps the customer to know in advance that where is the blocking port in the network based on the Port priority settings.