Purpose of STP
Summary
This topic explain common problems in a redundant, L2 switched network. Start learning CCNA 200-301 for free right now!!
Note: Welcome: This topic is part of Module 5 of the Cisco CCNA 2 course, for a better follow up of the course you can go to the CCNA 2 section to guide you through an order.
Table of Contents
Redundancy in Layer 2 Switched Networks
This topic covers the causes of loops in a Layer 2 network and briefly explains how spanning tree protocol works. Redundancy is an important part of the hierarchical design for eliminating single points of failure and preventing disruption of network services to users. Redundant networks require the addition of physical paths, but logical redundancy must also be part of the design. Having alternate physical paths for data to traverse the network makes it possible for users to access network resources, despite path disruption. However, redundant paths in a switched Ethernet network may cause both physical and logical Layer 2 loops.
Ethernet LANs require a loop-free topology with a single path between any two devices. A loop in an Ethernet LAN can cause continued propagation of Ethernet frames until a link is disrupted and breaks the loop.
Spanning Tree Protocol
Spanning Tree Protocol (STP) is a loop-prevention network protocol that allows for redundancy while creating a loop-free Layer 2 topology. IEEE 802.1D is the original IEEE MAC Bridging standard for STP.
Click Play in the figure to view an animation of STP in action.
STP Normal Operation
STP Recalculation
Click Play in the next figure to view an animation of STP recalculation when a failure occurs.
STP Compensates for Network Failure
Issues with Redundant Switch Links
Path redundancy provides multiple network services by eliminating the possibility of a single point of failure. When multiple paths exist between two devices on an Ethernet network, and there is no spanning tree implementation on the switches, a Layer 2 loop occurs. A Layer 2 loop can result in MAC address table instability, link saturation, and high CPU utilization on switches and end-devices, resulting in the network becoming unusable.
Unlike the Layer 3 protocols, IPv4 and IPv6, Layer 2 Ethernet does not include a mechanism to recognize and eliminate endlessly looping frames. Both IPv4 and IPv6 include a mechanism that limits the number of times a Layer 3 networking device can retransmit a packet. A router will decrement the TTL (Time to Live) in every IPv4 packet, and the Hop Limit field in every IPv6 packet. When these fields are decremented to 0, a router will drop the packet. Ethernet and Ethernet switches have no comparable mechanism for limiting the number of times a switch retransmits a Layer 2 frame. STP was developed specifically as a loop prevention mechanism for Layer 2 Ethernet.
Layer 2 Loops
Without STP enabled, Layer 2 loops can form, causing broadcast, multicast and unknown unicast frames to loop endlessly. This can bring down a network within a very short amount of time, sometimes in just a few seconds. For example, broadcast frames, such as an ARP Request are forwarded out all of the switch ports, except the original ingress port. This ensures that all devices in a broadcast domain are able to receive the frame. If there is more than one path for the frame to be forwarded out of, an endless loop can result. When a loop occurs, the MAC address table on a switch will constantly change with the updates from the broadcast frames, which results in MAC database instability. This can cause high CPU utilization, which makes the switch unable to forward frames.
Broadcast frames are not the only type of frames that are affected by loops. Unknown unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device. An unknown unicast frame is when the switch does not have the destination MAC address in its MAC address table and must forward the frame out all ports, except the ingress port.
Click Play in the figure to view the animation. When the animation pauses, read the text describing the action. The animation will continue after the short pause.
Broadcast Storm
A broadcast storm is an abnormally high number of broadcasts overwhelming the network during a specific amount of time. Broadcast storms can disable a network within seconds by overwhelming switches and end devices. Broadcast storms can be caused by a hardware problem such as a faulty NIC or from a Layer 2 loop in the network.
Layer 2 broadcasts in a network, such as ARP Requests are very common. A Layer 2 loop is likely to have immediate and disabling consequences on the network. Layer 2 multicasts are typically forwarded the same way as a broadcast by the switch. So, although IPv6 packets are never forwarded as a Layer 2 broadcast, ICMPv6 Neighbor Discovery uses Layer 2 multicasts.
Click Play in the figure to view an animation that shows the increasingly adverse effects of a loop as the broadcast and unknown unicast frames continue to propagate indefinitely in a broadcast storm.
A host caught in a Layer 2 loop is not accessible to other hosts on the network. Additionally, due to the constant changes in its MAC address table, the switch does not know out of which port to forward unicast frames. In the previous animation, the switches will have the incorrect ports listed for PC1. Any unknown unicast frame destined for PC1 loops around the network, just as the broadcast frames do. More and more frames looping around the network eventually creates a broadcast storm.
To prevent these issues from occurring in a redundant network, some type of spanning tree must be enabled on the switches. Spanning tree is enabled, by default, on Cisco switches to prevent Layer 2 loops from occurring.
The Spanning Tree Algorithm
STP is based on an algorithm invented by Radia Perlman while working for Digital Equipment Corporation, and published in the 1985 paper “An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN.” Her spanning tree algorithm (STA) creates a loop-free topology by selecting a single root bridge where all other switches determine a single least-cost path.
Without the loop prevention protocol, loops would occur rendering a redundant switch network inoperable.
Click each button for an explanation of how STA creates a loop-free topology.
- STA Scenario Topology
- Select the Root Bridge
- Block Redundant Paths
- Loop-Free Topology
- Link Failure Causes Recalculation
This STA scenario uses an Ethernet LAN with redundant connections between multiple switches.
The spanning tree algorithm begins by selecting a single root bridge. The figure shows that switch S1 has been selected as the root bridge. In this topology, all links are equal cost (same bandwidth). Each switch will determine a single, least cost path from itself to the root bridge.
Note: The STA and STP refers to switches as bridges. This is because in the early days of Ethernet, switches were referred to as bridges.
STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop, as shown in the figure. When a port is blocked, user data is prevented from entering or leaving that port. Blocking the redundant paths is critical to preventing loops on the network.
Switches S4, S5, and S8 have blocked redundant paths to the root bridge.
A blocked port has the effect of making that link a non-forwarding link between the two switches, as shown in the figure. Notice that this creates a topology where each switch has only a single path to the root bridge, similar to branches on a tree that connect to the root of the tree.
Each switch now has just one forwarding path to the root bridge.
The physical paths still exist to provide redundancy, but these paths are disabled to prevent the loops from occurring. If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the redundant path to become active. STP recalculations can also occur any time a new switch or new inter-switch link is added to the network.
The figure shows a link failure between switches S2 and S4 causing STP to recalculate. Notice that the previously redundant link between S4 and S5 is now forwarding to compensate for this failure. There is still only one path between every switch and the root bridge.
STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed “blocking-state” ports. The switches running STP are able to compensate for failures by dynamically unblocking the previously blocked ports and permitting traffic to traverse the alternate paths.
Packet Tracer – Investigate STP Loop Prevention
In this Packet Tracer activity, you will complete the following objectives:
- Create and configure a simple three switch network with STP.
- View STP operation
- Disable STP and view operation again.
Glossary: If you have doubts about any special term, you can consult this computer network dictionary.
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