The Experiment of Spanning Tree Report

In this experiment, I implemented spanning tree protocol which is a layer 2 protocol. Spanning tree protocol was invented by Radia Perlman to avoid loops in the network. Spanning tree protocol helps to avoid loops in case of redundant paths. Loops are avoided in spanning tree protocol because they are leather in a network. Spanning tree protocol are used in places where we want redundant links but not loops. In case of a network failure redundant links can play the crucial role of a backup in the network. With spanning tree protocol used on the switch, if there is a failure in the primary link it automatically turns to the backup links so that the network is never down and is ready to use always.

Spanning tree protocol builds a loop free topology for ethernet networks. There is no Time to Leave logic (TTL) in ethernet networks because of this loop is created or broadcast storm happens and thus spanning tree is used. Thus, it’s basic role is to avoid such loops and the broadcast storms (radiations) and provide backup links to avoid failures. Spanning tree protocol (STP) creates spanning tree within a network of connected layer 2 switches and it will disable the links that are not part of the spanning tree. Thus, it leaves behind only one single active part between the two network nodes.

In this experiment, we also learned about the root bridge of the spanning tree. The root bridge is decided with the bridge having the lowest bridge ID. All the bridge ID have a Mac address and a priority number. The default bridge ID is 32768 and it is in multiples of 4096. When we see the 2 bridge ID’s, we first compare their priority and then the Mac addresses are compared (if and only if their priorities are equal). The root bridge is decided when the bridge has lowest priority and lowest Mac address. In my topology, switch 1 was made the root port since it has the lowest priority number and the lowest Mac address.

I practiced 3 basic commands which were ‘show spanning-tree root’, ‘show spanning-tree blockedports’ ‘show spanning-tree brief’. When the show spanning tree brief operation was performed, fastethernet 1/0 from switch 3 was blocked and so it couldn’t reach the root bridge. During this process, the switch sends a message which in turn helps the formation of loop free network. This loop free network is know as Bridge Protocol Data Units (BPDU). It is necessary that each bridge should have enough information and to confirm that some special data frames are being which are called as bridge protocol data units to exchange information about bridge ID’s and root path cost.