Spanning Tree Protocol

STP Overview

In Layer 2 domain, redundancy is an important case. To provide redundant links, multiple connections are done between switches. But this redundancy mechanism can cause an undesirable situation that is called “L2 Loop”. To provide L2 redundancy and to avoid L2 loops, Spanning Tree Protocol (STP) has been developed. In this lesson we will focus STP Overview. In another lesson we will focus STP Configuration. You can also view Spanning Tree Protocol details lesson.

During the development period, various versions of STP has been introduced. Different standards has been created beside Cisco specific versions. These STP versions are :

• STP (Spanning Tree Protocol) – 802.1D
• RSTP(Rapid Spanning Tree Protocol) – 802.1W
• PVST+(Per VLAN Spanning Tree Plus) – Cisco Proprietary
• Rapid PVST+(Rapid Per VLAN Spanning Tree Plus) – Cisco Proprietary
• MST(Multiple Spanning Tree) – 802.1s

In this article, mainly we will focus on STP (802.1D).

Redundant Layer 2 Problems

Redundancy is an important term for a network. Layer 2 Redundancy is also important but need more caution. Without a mechanims like STP, Layer 2 Redundancy mechanisms can cause the below problems:

• Layer 2 Loops and Broadcast Storms
• MAC Address Instability
• Duplicate Frame Transmission

Let’s talk about each of these Layer 2 Redundacy problems.

Layer 2 Loops and Broadcast Storms

Layer 2 Loops can occur when there is a multiple available Layer 2 link and this links also send and receive frames more than one. And because of the fact that, there is no TTL mechanims in Layer 2, Layer 2 Loops occurs. As you remember, in Layer 3, there is a TTL mechanism.

Think about the below topology:

Here, when Switch A receives a frame from Segment 1 and send it to the Segment 2, Switch B also can learn it from Segment 2. And it also sends this frame to Segment 1 as if it is being done first time. So, frames are doubled and a Layer 2 Loop Occured. And this Layer 2 loop causes a Broadcast Storm. An infinite frame send/receive process occurs.

One loop in a Layer 2 domain can cause one more Alyer 2 loops even if the siwtched network is a large network.

MAC Address Instability

This Loops also cause a MAC Instability issue. As you remember, source address of a frame is important for MAC address types. These tables are created with source ports and MAC addresses. During a Loop, a frame can be received from different sources. So MAC Instability problem occurs. Here, a MAC table can have multiple MAC addresses for the same ports. This is an undefinable case for switches.

Multiple Frame Transmission

Multiple Frame Transmission is the problem of multiple, unnecessariliy transfer of Layer 2 frames. Because of there are multiple paths to a destination, same frame can be sent through different ways and this causes multiple frame transmission.

How STP Avoid Layer 2 Loops?

Now you can think that, we will both want to have redundant links and we want to avoid from Layer 2 Loops. How can we achieve this? We will achieve this with the Spanning Tree Protocol(STP).

Spanning Tree Protocol (STP) has a mechanims to avoid Layer 2 Loops. With its mechanism, it checked all the available links. It determiens primary and backups of redundant links. According to some algorithms, some of the links pruned and only one available link become active at the same time to a destination. We can say that Spanning Tree Protocol (STP) mechanims basically prunes the unnecessary links at that time. If this link become offline because of a problem, then STP make another link as active. And the communication continue with another available way. At a time, only one link is active with STP.

BPDU

BPDU is the abbreviation of Bridge Protocol Data Unit. BPDU is the messages used between switches. BPDUs are sent and received from the ports. But blocking ports are only receive them to determine any failure etc. They do not sent BPDUs.

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