Spanning Tree Protocol

Spanning Tree Protocol
7.2.2 This page will explain how STP can be used to create a loop free network.
Ethernet bridges and switches can implement the IEEE 802.1d Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network. 
Shortest path is based on cumulative link costs. Link costs are based on the speed of the link. 
The Spanning-Tree Protocol establishes a root node called the root bridge. The Spanning-Tree Protocol constructs a topology that has one path for every node on the network. This tree originates from the root bridge. Redundant links that are not part of the shortest path tree are blocked.
It is because certain paths are blocked that a loop free topology is possible. Data frames received on blocked links are dropped.
The Spanning-Tree Protocol requires network devices to exchange messages to detect bridging loops. Links that will cause a loop are put into a blocking state.
Switches send messages called the bridge protocol data units (BPDUs) to allow the formation of a loop free logical topology. BPDUs continue to be received on blocked ports. This ensures that if an active path or device fails, a new spanning-tree can be calculated.
BPDUs contain information that allow switches to perform specific actions:

• Select a single switch that will act as the root of the spanning-tree.
• Calculate the shortest path from itself to the root switch.
• Designate one of the switches as the closest one to the root, for each LAN segment. This switch is called the designated switch. The designated switch handles all communication from that LAN segment towards the root bridge.
• Choose one of its ports as its root port, for each non-root switch. This is the interface that gives the best path to the root switch.
• Select ports that are part of the spanning-tree. These ports are called designated ports. Non-designated ports are blocked. 

The Interactive Media Activity will teach students about STP.
The next page will describe the features of a spanning-tree network.

Spanning-Tree Protocol / Redundant topology and spanning tree

Spanning-Tree Protocol / 
Redundant topology and spanning tree
7.2.1 This page will teach students how to create a loop free logical topology.
Redundant network topologies are designed to ensure that networks continue to function in the presence of single points of failure. Work is interrupted less often for users because the network continues to function. Any interruptions that are caused by a failure should be as short as possible.
Reliability is increased by redundancy. A network that is based on switches or bridges will introduce redundant links between those switches or bridges to overcome the failure of a single link. These connections introduce physical loops into the network. These bridging loops are created so if one link fails another can take over the function of forwarding traffic.
When the destination of the traffic is unknown to a switch, it floods traffic out all ports except the port that received the traffic. Broadcasts and multicasts are also forwarded out every port except the port that received the traffic. This traffic can be caught in a loop. 
In the Layer 2 header, there is no Time To Live (TTL) value. If a frame is sent into a Layer 2 looped topology of switches, it can loop forever. This wastes bandwidth and makes the network unusable.
At Layer 3, the TTL is decremented and the packet is discarded when the TTL reaches 0. This creates a dilemma. A physical topology that contains switching or bridging loops is necessary for reliability, yet a switched network cannot have loops.
The solution is to allow physical loops, but create a loop free logical topology. For this logical topology, traffic destined for the server farm attached to Cat-5 from any user workstation attached to Cat-4 will travel through Cat-1 and Cat-2. This will happen even though there is a direct physical connection between Cat-5 and Cat-4.
The loop free logical topology created is called a tree. This topology is a star or extended star logical topology. This topology is the spanning-tree of the network. It is a spanning-tree because all devices in the network are reachable or spanned.
The algorithm used to create this loop free logical topology is the spanning-tree algorithm. This algorithm can take a relatively long time to converge. A new algorithm called the rapid spanning-tree algorithm was developed to reduce the time for a network to compute a loop free logical topology.
The next page will discuss STP.

Media access control database instability

Media access control database instability
7.1.6 This page will explain how incorrect information can be forwarded in a redundant switched network.
In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port. In this example the MAC address of Router Y is not in the MAC address table of either switch.
Host X sends a frame directed to Router Y. Switches A and B learn the MAC address of Host X on port 0.
The frame to Router Y is flooded on port 1 of both switches. Switches A and B receive this information on port 1 and incorrectly learn the MAC address of Host X on port 1. When Router Y sends a frame to Host X, Switch A and Switch B also receive the frame and will send it out port 1. This is unnecessary, but the switches have incorrectly learned that Host X is on port 1.
In this example the unicast frame from Router Y to Host X will be caught in a loop.
This page concludes this lesson. The next lesson will describe the Spanning-Tree Protocol (STP). The first page will discuss physical and logical loops in a redundant network

Broadcast storms / Multiple frame transmissions

Broadcast storms
7.1.4 page will explain the effects of broadcasts and multicasts in a switched network.
Broadcasts and multicasts can cause problems in a switched network.
Multicasts are treated as broadcasts by the switches. Broadcast and multicast frames are flooded out all ports, except the one on which the frame was received.
If Host X sends a broadcast, like an ARP request for the Layer 2 address of the router, then Switch A will forward the broadcast out all ports. Switch B is on the same segment and also forwards all broadcasts. Switch B receives all the broadcasts that Switch A forwarded and Switch A receives all the broadcasts that Switch B forwarded. Switch A forwards the broadcasts received from Switch B. Switch B forwards the broadcasts received from Switch A. 
The switches continue to propagate broadcast traffic over and over. This is called a broadcast storm. This broadcast storm will continue until one of the switches is disconnected. Since broadcasts require time and network resources to process, they reduce the flow of user traffic. The network will appear to be down or extremely slow.
The next page will discuss multiple frame transmissions.
A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems.
The next page will discuss broadcast storms.

Multiple frame transmissions 7.1.5 page will explain the effects of broadcasts and multicasts in a switched network.
Broadcasts and multicasts can cause problems in a switched network.
Multicasts are treated as broadcasts by the switches. Broadcast and multicast frames are flooded out all ports, except the one on which the frame was received.
If Host X sends a broadcast, like an ARP request for the Layer 2 address of the router, then Switch A will forward the broadcast out all ports. Switch B is on the same segment and also forwards all broadcasts. Switch B receives all the broadcasts that Switch A forwarded and Switch A receives all the broadcasts that Switch B forwarded. Switch A forwards the broadcasts received from Switch B. Switch B forwards the broadcasts received from Switch A.
The switches continue to propagate broadcast traffic over and over. This is called a broadcast storm. This broadcast storm will continue until one of the switches is disconnected. Since broadcasts require time and network resources to process, they reduce the flow of user traffic. The network will appear to be down or extremely slow.
The next page will discuss multiple frame transmissions.

Reduntant Topoligies / Redundant switched topologies

Redundant Topoligies
7.1.2 This page will explain the concept and benefits of a redundant topology.
A goal of redundant topologies is to eliminate network outages caused by a single point of failure. All networks need redundancy for enhanced reliability.
A network of roads is a global example of a redundant topology. If one road is closed for repair, there is likely an alternate route to the destination. 
Consider a community separated by a river from the town center. If there is only one bridge across the river, there is only one way into town. The topology has no redundancy. 
If the bridge is flooded or damaged by an accident, travel to the town center across the bridge is impossible. 
A second bridge across the river creates a redundant topology. The suburb is not cut off from the town center if one bridge is impassable.   
The next page will describe redundant switched topologies.
Redundant switched topologies
7.1.3 This page will explain how switches operate in a redundant topology.
Networks with redundant paths and devices allow for more network uptime. Redundant topologies eliminate single points of failure. If a path or device fails, the redundant path or device can take over the tasks of the failed path or device. 
If Switch A fails, traffic can still flow from Segment 2 to Segment 1 and to the router through Switch B.
Switches learn the MAC addresses of devices on their ports so that data can be properly forwarded to the destination. Switches flood frames for unknown destinations until they learn the MAC addresses of the devices. Broadcasts and multicasts are also flooded. 
A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems.
The next page will discuss broadcast storms.

Redundant Topologies / Redundancy

Redundancy
7.1.1 This page will explain how redundancy can improve network reliability and performance.
Many companies and organizations increasingly rely on computer networks for their operations. Access to file servers, databases, the Internet, intranets, and extranets is critical for successful businesses. If the network is down, productivity and customer satisfaction decline.
Increasingly, companies require continuous network availability, or uptime. 100 percent uptime is perhaps impossible, but many organizations try to achieve 99.999 percent, or five nines, uptime. Extremely reliable networks are required to achieve this goal. This is interpreted to mean one hour of downtime, on average, for every 4,000 days, or approximately 5.25 minutes of downtime per year. To achieve such a goal requires extremely reliable networks.
Network reliability is achieved through reliable equipment and network designs that are tolerant to failures and faults. Networks should be designed to reconverge rapidly so that the fault is bypassed.
Figure illustrates redundancy. Assume that a car must be used to get to work. If the car has a fault that makes it unusable, it is impossible to use the car to go to work until it is repaired.
On average, if the car is unuseable due to failure one day out of ten, the car has ninety percent usage. Therefore, reliability is also 90 percent.
A second car will improve matters. There is no need for two cars just to get to work. However, it does provide redundancy, or backup, in case the primary vehicle fails. The ability to get to work is no longer dependent on a single car.
Both cars may become unusable simultaneously, one day in every 100. The second car raises reliability to 99 percent. 
The next page will discuss redundant topologies

Module 7: Spanning-Tree Protocol : Overview

Spanning-Tree Protocol (Overview)

Redundancy in a network is critical. It allows networks to be fault tolerant. Redundant topologies protect against network downtime, or nonavailability. Downtime can be caused by the failure of a single link, port, or network device. Network engineers are often required to balance the cost of redundancy with the need for network availability. Redundant topologies based on switches and bridges are susceptible to broadcast storms, multiple frame transmissions, and MAC address database instability. These problems can make a network unusable. Therefore, redundancy should be carefully planned and monitored. Switched networks provide the benefits of smaller collision domains, microsegmentation, and full duplex operation. Switched networks provide better performance. Redundancy in a network is required to protect against loss of connectivity due to the failure of an individual component. However, this provision can result in physical topologies with loops. Physical layer loops can cause serious problems in switched networks. The Spanning-Tree Protocol is used in switched networks to create a loop free logical topology from a physical topology that has loops. Links, ports, and switches that are not part of the active loop free topology do not forward data frames. The Spanning-Tree Protocol is a powerful tool that gives network administrators the security of a redundant topology without the risk of problems caused by switching loops. This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.   Students who complete this module should be able to perform the following tasks: Define redundancy and its importance in networking Describe the key elements of a redundant network topology Define broadcast storms and describe their impact on switched networks Define multiple frame transmissions and describe their impact on switched networks Identify causes and results of MAC address database instability Identify the benefits and risks of a redundant topology Describe the role of spanning-tree in a redundant-path switched network Identify the key elements of spanning-tree operation Describe the process for root bridge election List the spanning-tree states in order Compare Spanning-Tree Protocol and Rapid Spanning-Tree Protocol