Redundancy is a crucial case for the Layer 2 domain. Multiple connections are established between switches to provide redundant links. However, this redundancy mechanism can result in an undesirable condition known as “L2 Loop.” Spanning Tree Protocol (STP) has been created to provide L2 redundancy and prevent L2 loops. This lesson will concentrate on STP Overview. In a subsequent lesson, we will examine STP Configuration.
During the period of development, numerous versions of STP were introduced. Different standards have been developed in addition to Cisco-specific variants. These STP variants include:
- STP – Spanning Tree Protocol – 802.1D
- RSTP – Rapid Spanning Tree Protocol) – 802.1W
- PVST+ – Per VLAN Spanning Tree +
- Rapid PVST+ – Rapid Per VLAN Spanning Tree +
- MST – (Multiple Spanning Tree) – 802.1s
In this article we will focus solely on STP (802.1D).
Redundant Layer 2 Issues
Redundancy is an essential network term. Layer 2 Redundancy is also essential, but requires more care. Without a STP-like mechanism, Layer 2 Redundancy mechanisms can cause the following issues:
- Loops on Layer 2 and Broadcast Storms
- MAC Address Inconsistency
- Repetition of Frame Transmission
Let’s discuss about each of these Layer 2 Redundancy issues.
Loops on Layer 2 and Broadcast Storms
Layer 2 Loops can occur when multiple Layer 2 links are available and these links send and receive multiple frames. As a result of the absence of a TTL mechanism in Layer 2, Layer 2 Loops occur. As you may recall, Layer 3 contains a TTL mechanism.
Think about the topology below:
When Switch A receives a frame from Segment 1 and forwards it to Segment 2, Switch B can also learn the frame from Segment 2. In addition, this frame is sent to Segment 1 as if it were being sent for the first time. Therefore, frames are duplicated and a Loop on Layer 2 occurs. In addition, this Layer 2 loop results in a Broadcast Storm. There is an infinite frame send/receive process.
Even if the switched network is a large network, a single loop in a Layer 2 domain can cause additional Layer 2 loops.
MAC Address Inconsistency
These Loops also contribute to MAC Instability issues. As you may recall, source address is crucial for MAC address types. Source ports and MAC addresses are used to populate these tables. A frame can be received from multiple sources during a Loop. Thus, the MAC Instability issue occurs. In this instance, a MAC table may contain multiple MAC addresses for the same ports. This is an indefinable switch case.
Repetition of Frame Transmission
Repetion of Frame Transmission refers to the issue of redundant Layer 2 frame transfer. Because there are multiple paths to a destination, the same frame can be transmitted in multiple ways, resulting in multiple frame transmissions.
