Which spanning tree mode

Now, we have two instances of spanning tree running. This means that every VLAN has a single instance of spanning tree. For example, we want the traffic from the Sales and the Management departments to be forwarded to their root bridge at SW2 and blocked on SW1. Also, the traffic from the Engineering and the Production departments will be forwarded to their root bridge at SW1, and SW2 will be in a blocked state.

There will be a total of four spanning-tree instances running, as we have four VLANs in the network. Assuming that we have VLANs in our network, we will also have spanning-tree instances. It would be consuming more resources as compared to grouping them like in MSTP. By providing other physical paths for data to travel through the network. In a switched Ethernet network, redundant paths can trigger logical and physical layer 2 loops. The Spanning Tree Protocol is an xloop-prevention network protocol.

It creates a loop-free Layer 2 topology thus allowing redundancy. Using this topology information, we can determine the most efficient loop-free path through the switched network. A root switch election occurs when all switches behave as if they are the root switch at first. They continue to do so until they receive traffic from a superior switch switch priority determines. This is known as a root switch election. On running the command the network administrator can see the priority of the switch and the value it has been set to.

The switch which has the lowest value becomes the root switch. The network administrator can change the switch priority that we will see when we begin the configurations. Another thing to bear in mind is that depending on the STP mode used, the network may have multiple root switches. Image source. Following the election of the root switch, each port is assigned a role based on its location within the STP topology. To avoid loops, all ports with the alternative or backup STP roles are blocked.

This is done until the best route has been determined and every port is allocated a role. Each activated port on a switch participates in STP. And each of these ports must go through an interface state phase before being allowed to forward traffic. Figure 1 depicts the The time it takes for a port to convert and the process used to transition have both improved since the RSTP was introduced. This allows a switched network to start routing traffic faster and without unnecessary delays.

This was a complaint about the We click on the switch in our cisco packet tracer and enter the commands as follows after we have created our topology:. Although the network might be able to decide this on its own, the election would simply be a matter of who has the lowest MAC address. The switch with the lowest bridge ID is selected as the root switch in the process of a root switch election. Using the commands above, we have enabled the spanning tree protocol STP on our switches and selected the root switch in our topology.

Which will determine the best route to use in the process of data transversing across the network. All other decisions in the network, such as which port to block and which port to put in forwarding mode, are made from the perspective of this root bridge. A switched environment, which is different from a bridge environment, most likely deals with multiple VLANs.

When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. The roots for the different VLANs can all reside in a single switch or in various switches. You can choose the root switch, or you can let the switches decide, which is risky. If you do not control the root selection process, there can be suboptimal paths in your network. All the switches exchange information for use in the root switch selection and for subsequent configuration of the network.

Bridge protocol data units BPDUs carry this information. Each switch compares the parameters in the BPDU that the switch sends to a neighbor with the parameters in the BPDU that the switch receives from the neighbor. In the STP root selection process, less is better. Before you configure STP, select a switch to be the root of the spanning tree. This switch does not need to be the most powerful switch, but choose the most centralized switch on the network.

All data flow across the network is from the perspective of this switch. Also, choose the least disturbed switch in the network. The backbone switches often serve as the spanning tree root because these switches typically do not connect to end stations. Also, moves and changes within the network are less likely to affect these switches. After you decide on the root switch, set the appropriate variables to designate the switch as the root switch.

The only variable that you must set is the bridge priority. If the switch has a bridge priority that is lower than all the other switches, the other switches automatically select the switch as the root switch. You can also issue the set spantree portfast command, on a per-port basis. When you enable the portfast variable on a port, the port immediately switches from blocking mode to forwarding mode. However, do not use this command when you have switch-to-switch connection.

In this case, the command can result in a loop. The to second delay that occurs during the transition from blocking to forwarding mode prevents a temporal loop condition in the network when you connect two switches. This section lists rules for how STP works. When the switches first come up, they start the root switch selection process. The switches then agree on which switch is the root switch. The switch with the lowest bridge ID in the network wins this election process.

After the root switch identification, the switches adhere to these rules:. Note: In some corner cases, which involve self-looped ports, there is an exception to this rule. Next, each switch determines the best path to get to the root.

The switches determine this path by a comparison of the information in all the BPDUs that the switches receive on all ports. The switch uses the port with the least amount of information in the BPDU in order to get to the root switch; the port with the least amount of information in the BPDU is the root port. After a switch determines the root port, the switch proceeds to rule 2. In addition, the switches on each LAN segment communicate with each other to determine which switch is best to use in order to move data from that segment to the root bridge.

This switch is called the designated switch. The rule only applies to ports that connect to other bridges or switches. STP does not affect ports that connect to workstations or PCs. These ports remain forwarded.

The other VLAN parts of a trunk link can forward traffic normally. Note: By default, spanning tree runs on every port.