CCNA RnS Article #29: What STP Does?

CCNA RnS Article #29: What Spanning Tree Protocol Does?

CCNA Basic Lab
Fig 1.1- CCNA Basic Lab

In this article, we are going to talk about what Spanning Tree Protocol does. This article is the second part of the previous one where we discussed
why STP.

Broadcast storm, MAC table inconsistency, and multiple copies of the same frame are some of the common problems in a network with redundant paths without STP. Let’s start with what STP does to prevent these issues.

STP prevents a loop in a network by placing switch ports either in a forwarding state or a blocking state. Only ports that are in forwarding state, receive and send traffic. Interfaces that are in a blocking state don’t forward traffic and learn the MAC address of received frames. In short blocking ports don’t process user traffic.

Let’s see our previous topology where broadcast storms were observed and with STP what happens that prevents the worse situation.

Figure 1: STP Block Port

With STP enabled, the above network responds to broadcast or unknown unicast packets differently. PC-1 sends a broadcast Switch 1 receives the same and follows the basic switching logic – that is to flood the broadcast frame out to other ports except where it is received. The same broadcast frame is received by Switch 2 and Switch 3. However, Switch 3 does not forward the broadcast frame on the fa0/0 interface as it is blocked by STP.

The same goes true with the broadcast frame from Switch 2 to Switch 3, it will be ignored as it received on fa0/0 as it is in the blocking state.

In a converged state (normal operation), the link between Switch 2 and Switch 3 will not be utilized for traffic forwarding. It may be a small drawback of using STP as it disables some of the ports in the network. But in case any connection failed in the topology, blocking port transition to the forwarding state and start forwarding the packet. So, you have high availability with redundant circuits.

What goes behind the scene when deciding which ports are in the forwarding state and which are to be in the blocking state is the main topic to understand STP. I believe it is clear by now what is done by STP to break a loop [blocking some of the ports in the network]. We’ll be covering the same in our upcoming articles. Hope you find this informative!

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