Moxa IKS-6728A-4GTXSFP-24-24-T [23/37] Stp calculation

Managed Ethernet Switch Redundancy Protocol (UI 2.0) STP/RSTP/MSTP

5-4

STP Calculation

The first step of the STP process is to perform calculations. During this stage, each bridge on the network

transmits BPDUs. The following items will be calculated:

• Which bridge should be the Root Bridge. The Root Bridge is the central reference point from which the

network is configured.

• The Root Path Costs for each bridge. This is the cost of the paths from each bridge to the Root Bridge.

• The identity of each bridge’s Root Port. The Root Port is the port on the bridge that connects to the Root

Bridge via the most efficient path. In other words, the port connected to the Root Bridge via the path with

the lowest Root Path Cost. The Root Bridge, however, does not have a Root Port.

• The identity of the Designated Bridge for each LAN segment. The Designated Bridge is the bridge with the

lowest Root Path Cost from that segment. If several bridges have the same Root Path Cost, the one with the

lowest Bridge Identifier becomes the Designated Bridge. Traffic transmitted in the direction of the Root

Bridge will flow through the Designated Bridge. The port on this bridge that connects to the segment is

called the Designated Bridge Port.

STP Configuration

After all of the bridges on the network agree on the identity of the Root Bridge, and all other relevant

parameters have been established, each bridge is configured to forward traffic only between its Root Port and

the Designated Bridge Ports for the respective network segments. All other ports are blocked, which means

that they will not be allowed to receive or forward traffic.

STP Reconfiguration

Once the network topology has stabilized, each bridge listens for Hello BPDUs transmitted from the Root Bridge

at regular intervals. If a bridge does not receive a Hello BPDU after a certain interval (the Max Age time), the

bridge assumes that the Root Bridge, or a link between itself and the Root Bridge, has ceased to funtion. This

will trigger the bridge to reconfigure the network to account for the change. If you have configured an SNMP

trap destination, the first bridge to detect the change will send out an SNMP trap when the topology of your

network changes.

Differences between STP, RSTP, and MSTP

RSTP is similar to STP, but includes additional information in the BPDUs that allow each bridge to confirm that

it has taken action to prevent loops from forming when it decides to enable a link to a neighboring bridge.

Adjacent bridges connected via point-to-point links will be able to enable a link without waiting to ensure that

all other bridges in the network have had time to react to the change. The main benefit of RSTP is that the

configuration decision is made locally rather than network-wide, allowing RSTP to carry out automatic

configuration and restore a link faster than STP.

STP and RSTP spanning tree protocols operate without regard to a network’s VLAN configuration, and maintain

one common spanning tree throughout a bridged network. Thus, these protocols map one loop-free, logical

topology on a given physical topology. MSTP uses VLANs to create multiple spanning trees in a network, which

significantly improves network resource utilization while maintaining a loop-free environment.