Issues without having STP

Issues without having STP

We need to remember first what is STP

Spanning Tree  (STP) is a Layer 2 protocol that runs on bridges and switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network.

Redundant links can cause a lot of problems including:

1. Broadcast storm
2. Trashing out the Mac table
3. Multiple copies of Ethernet frames 

STP prevents physical networking loops from occurring by identifying those redundant links in the network, blocking some of them and making a logical loop free network


In the following example, Host A is ARPing for the MAC address of B. Because the switches have redundant links, the broadcast will continue to get propagated until the network can no longer function due to high bandwidth utilization of the links and high processor utilization on the hosts.









  STP chooses a point of reference called the Root Bridge. By default the bridge with the lowest Bridge ID is chosen as the Root. The Bridge ID is a 64-bit field made up of the priority (the default is 32768) and the MAC address. If the default priority has not been changed, the Root will be chosen based on the lowest MAC address.

The Root Bridge then sends Bridge Protocol Data Units (BPDUs) out all of its designated (forwarding) ports. If a bridge is not the Root (called a Non-Root Bridge), it will receive regular BPDU on the port that is closest to the Root, called the Root Port.

 Please based, on our previous explanation try to understand this topology

What we see here:

1. Switch A is the Root bridge because it has the lowest hexadecimal digit  Mac address 
2. Port fa0/14 and Fa0/17 on Switch A are the Designated ports  for being closer to the root bridge
3. Port Fa014 on switch B and Fa0/14 on Switch C (non-root) are closer to the root bridge..them they are the Root port