Why spanning tree is important

Also, should spanning tree be enabled on all ports? Not configuring spanning tree at all As I said, spanning tree is a good thing. But for some reason, a lot of switch vendors disable it by default. So out of the box, you might have to enable the protocol. STP prevents loops by blocking one or more of the links. If one of the links in use goes down, then it would fail over to a previously blocked link. The Spanning Tree Protocol STP is widely used for network redundancy and resilience in Ethernet networks, in spite of its well-known flaws and limitations.

The Spanning Tree Protocol STP is responsible for identifying links in the network and shutting down the redundant ones, preventing possible network loops. In order to do so, all switches in the network exchange BPDU messages between them to agree upon the root bridge.

What is Spanning Tree with example? For example, suppose we start with this graph: We can remove edges until we are left with a tree: the result is a spanning tree. Clearly, a spanning tree will have V -1 edges, like any other tree. What is a spanning? A spanning tree is a subset of Graph G, which has all the vertices covered with minimum possible number of edges.

By this definition, we can draw a conclusion that every connected and undirected Graph G has at least one spanning tree. Spanning tree is basically used to find a minimum path to connect all nodes in a graph. Let us understand this through a small example. Consider, city network as a huge graph and now plans to deploy telephone lines in such a way that in minimum lines we can connect to all city nodes. This is where the spanning tree comes into picture. In a weighted graph, a minimum spanning tree is a spanning tree that has minimum weight than all other spanning trees of the same graph.

In real-world situations, this weight can be measured as distance, congestion, traffic load or any arbitrary value denoted to the edges. Kruskal's Algorithm. 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. The spanning tree feature cannot be turned off in switches on a per-port basis.

Extreme care should be taken whenever you disable spanning tree because this creates Layer 2 loops within the network.