It also blocks all the other higher cost links to avoid looping in the network. Once link costs are estimated, STP/RSTP determines the lowest cost connections from each designated switch to the root switch to determine the path with the lowest cost. For example, higher bandwidth (1000 Mbps) links that are directly connected/ close to the root are assigned a lower cost (and) lower bandwidth (10 Mbps) links that are multiple hops away from the root are assigned a higher cost. There is a certain cost assigned to every link. The other switches in the network are called designated switches. If the root switch fails, all the other switches repeat the election process and select a new root switch, from the available switches. If it is not selected manually, the switches will themselves elect a root switch.
Its possible (and recommended) to manually select the root switch. Generally, the root switch is elected by selecting the switch with the lowest bridge ID. In our above diagram, Switch C could be considered as the Root bridge as it is in the central location. The Root Switch is the Central reference Switch. It is with reference to the Root Switch that all the other switches determine their best cost path. Electing the Root:Įlecting the Root bridge/ Root Switch is one important process in Spanning Tree Protocol. Similarly, Switches C, D, E, F & G can also provide link redundancy if they are connected as shown in the above diagram & STP/RSTP enabled switches are employed. This provides for both link redundancy and switch redundancy in the network. If either L1 or L2 links fail, this link (元) automatically gets activated. Its very much connected to the Switches A & B and keeps listening to the network. If no Spanning Tree Protocol is applied in this scenario, there would be looping of data and hence broadcast congestion in the network.īut if STP enabled switches are employed, the link 元 is effectively blocked from transmitting any data. But there is another link (marked in red) that goes from Switch A to Switch B. Switch C is connected to Switch A & Switch B via direct links. For the moment, let us concentrate on the Switch A, B & C. They are connected and kept inactive by STP, so that, when the best path fails – the next best path can be achieved by activating the blocked links.Ĭonsider the above diagram. The blocked links are not always discarded. So, STP is used to identify the best path to the destination, and block all other paths (links). When there are two or more paths to the same destination, there is a danger of broadcast packets getting in to an infinite loop and hence causing congestion in the network. Network loops sometimes happen inadvertently, especially in large networks. Spanning Tree Protocol, and its improved versions, are required mainly for two reasons – To prevent network loops (due to multiple paths to the same destination) & to introduce redundancy in the link connections (if one link fails, the data is still routed through a different link/route). What is Spanning Tree Protocol (STP) and why is it required? In this article, we will try to understand the basic concepts of Spanning Tree Protocols and their implementation. STP is the short form for Spanning Tree Protocol & RSTP (Rapid Spanning Tree Protocol), MSTP (Multiple Spanning Tree Protocol) are all advanced/ improved implementations of STP.
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