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Which spanning tree standard supports only one root bridge so that traffic from all VLANs flows over the same path?

Which spanning tree standard supports only one root bridge so that traffic from all VLANs flows over the same path?

  • Rapid PVST
  • 802.1D
  • MST
  • PVST+

The correct answer is 802.1D, also known as the original Spanning Tree Protocol (STP). Here’s a detailed explanation of why this is the correct choice and a deeper dive into the technical details of STP and spanning tree protocols in general.

Spanning Tree Protocol (STP) Overview

The Spanning Tree Protocol (STP) was developed by Radia Perlman in 1985 and was later standardized in IEEE 802.1D. The primary purpose of STP is to prevent broadcast storms and network loops in Layer 2 networks, which can occur when multiple switches are interconnected in such a way that redundant paths exist.

Without STP, a loop in the network could result in data frames being endlessly circulated, consuming bandwidth and eventually overwhelming network devices. STP eliminates this issue by logically “blocking” redundant links while keeping one active path between any two devices. If the active path fails, STP recalculates the network topology to unblock one of the redundant links, providing failover capabilities.

How STP (802.1D) Works

Root Bridge Selection

The first step in establishing the spanning tree topology is the election of the root bridge. The root bridge serves as the central point for all spanning tree calculations, and traffic from all VLANs typically flows through this device. The root bridge is selected based on the Bridge ID (BID), which consists of two components: the bridge priority and the MAC address of the switch. By default, all switches have the same priority (32768), so the switch with the lowest MAC address becomes the root bridge.

Once the root bridge is elected, each non-root switch calculates the shortest path to the root bridge using the Root Path Cost, which is determined by the bandwidth of the links in the path. The port on each switch that offers the lowest cost to the root bridge is designated as the Root Port, and the remaining ports may be either Designated Ports (forwarding) or Blocked Ports (non-forwarding) to prevent loops.

One Root Bridge for All VLANs

In 802.1D, there is only one root bridge per network, regardless of the number of VLANs. This means that all VLANs share the same spanning tree topology, with traffic from all VLANs flowing through the same root bridge and, by extension, the same network paths. While this simplifies network design and operation, it also has limitations, as traffic from different VLANs cannot take advantage of redundant links, even if some links are underutilized.

For instance, imagine a network with two redundant links between two switches and two VLANs (VLAN 10 and VLAN 20). In 802.1D, only one of these links would be active, while the other would remain in a blocking state. Traffic from both VLANs would flow over the same link, potentially creating a bottleneck if one link becomes congested. The alternative links can only be used if the primary link fails.

This single-path design has some downsides, especially in large-scale or high-traffic environments where balancing traffic across multiple paths is desirable.

STP Convergence

Convergence is the process by which STP recalculates the network topology when a change (such as a link failure) occurs. In the original 802.1D STP, convergence can take a significant amount of time, typically around 30-50 seconds. This delay is due to the time it takes for the switches to transition between the following port states:

  • Blocking: The port does not forward any traffic.
  • Listening: The port listens for BPDU (Bridge Protocol Data Units) frames to determine the network topology but does not forward traffic.
  • Learning: The port starts learning MAC addresses but still does not forward traffic.
  • Forwarding: The port forwards traffic.

The long convergence time can be problematic in environments where quick recovery from failures is critical.

STP Enhancements

To address the limitations of 802.1D, several enhancements and alternative spanning tree protocols have been developed. These include:

1. PVST+ (Per VLAN Spanning Tree Plus):

PVST+ is a Cisco proprietary enhancement to STP. Unlike 802.1D, PVST+ creates a separate spanning tree instance for each VLAN. This allows traffic from different VLANs to utilize different network paths, thereby making better use of redundant links and improving traffic load balancing. For example, VLAN 10 could take one path, while VLAN 20 could take another.

While PVST+ provides better traffic distribution than 802.1D, it requires more processing power and memory on the switches since each VLAN must maintain its own spanning tree topology.

2. Rapid PVST+:

Rapid PVST+ is another Cisco proprietary protocol that combines the benefits of PVST+ with the faster convergence times of the Rapid Spanning Tree Protocol (RSTP, standardized in 802.1w). Like PVST+, it maintains a separate spanning tree instance for each VLAN but offers much quicker convergence compared to 802.1D.

3. MST (Multiple Spanning Tree):

MST, or Multiple Spanning Tree Protocol, is defined by the IEEE 802.1s standard. It allows multiple VLANs to be mapped to a single spanning tree instance, reducing the overhead associated with maintaining separate spanning trees for each VLAN, as in PVST+. MST is more efficient for larger networks where you have many VLANs and need a balance between traffic load balancing and resource consumption.

4. RSTP (Rapid Spanning Tree Protocol, 802.1w):

RSTP significantly improves upon the original STP by reducing convergence times from 30-50 seconds to just a few seconds (typically less than 6). It achieves this by making changes to the way port states are handled and by enabling immediate forwarding on edge ports. While RSTP is much faster, like 802.1D, it supports only one spanning tree instance for all VLANs, unless used with PVST+ or MST.

Advantages of 802.1D

Despite its limitations, 802.1D STP remains widely used in simpler, smaller networks for the following reasons:

  1. Simplicity: 802.1D provides a simple way to prevent Layer 2 loops with minimal configuration.
  2. Compatibility: Since 802.1D is an IEEE standard, it is supported on almost all networking equipment.
  3. Stability: In networks where redundancy is only a secondary concern, 802.1D’s single spanning tree instance offers a stable and predictable topology.
  4. Fewer Resources Required: Unlike PVST+ and MST, which require multiple instances of the spanning tree, 802.1D maintains only one spanning tree for the entire network, reducing the computational and memory burden on switches.

Conclusion

The IEEE 802.1D standard for Spanning Tree Protocol supports a single root bridge for the entire network, meaning all traffic from all VLANs flows over the same path. While this provides simplicity and stability, it limits the ability to optimize the use of redundant links. For larger networks with more complex VLAN setups, protocols like PVST+, MST, or Rapid PVST+ provide more flexibility and faster convergence.