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

Module 6 : Summary

Summary
This page summarizes the topics discussed in this module.
Switches are similar to routers. They have basic computer components including a CPU, RAM, and an operating system. There are several ports that are used to connect hosts and for management. LEDs on the front of the switch show the system status, RPS, port mode, and port status. When powered on, a switch performs POST automatically to verify that the switch functions correctly. HyperTerminal can be used to configure or check the status of a switch.
Another similarity to Cisco routers is the CLI. Enter a question mark (?) to access help. A list of available commands will display. Switches provide word help and command syntax help.
Switches and routers have the same command modes. User EXEC is the default and is indicated by the greater-than character (>). The enable command changes User EXEC to Privileged EXEC as indicated by the pound sign (#). Access to Privileged EXEC mode should be password protected to prevent unauthorized use. The configure command allows other command modes to be accessed.
Default data is provided when the switch is powered up for the first time. For management purposes, a switch is assigned an IP address. Use the show version command to verify the IOS version and the configuration register settings.
Once a switch is configured with an IP address and gateway, it can be accessed through a web-based interface. This allows for the configuration and management of the switch. This service can be accessed through a web browser with the IP address and port 80, the default port for http.
A switch dynamically learns and maintains thousands of MAC addresses. If frames with a previously learned address are not received, the MAC address entry is automatically discarded or aged out after 300 seconds. The command clear mac-address-table entered in the Privileged EXEC mode can be used to manually clear address tables.
A permanent MAC address assigned to an interface ensures that the MAC address will not be aged out automatically by the switch and to enhance security. The command mac-address-table static <mac-address of host > interface FastEthernet <Ethernet number > vlan <vlan name > can be used to configure a static MAC address. Use the no form of the command to remove it. The command show port security can be used to verify port security.
The switch name, IP address, default gateway, and line passwords should be configured on a new switch that is added to a network. When a host is moved from one port or switched to another, configurations that can cause unexpected behavior should be removed. Documentation should be maintained for the current configuration and backups to the server or a disk should be performed periodically. 

1900/2950 password recovery / 1900/2950 firmware upgrade

1900/2950 password recovery 
6.2.8 For security and management purposes, passwords must be set on the console and vty lines. An enable password and an enable secret password must also be set. These practices help ensure that only authorized users have access to the User and Privileged EXEC modes of the switch.
There will be circumstances where physical access to the switch can be achieved, but access to the User or Privileged EXEC mode cannot be gained because the passwords are not known or have been forgotten. 
In these circumstances, a password recovery procedure must be followed.
The Lab Activities will show students how to recover a password on a Catalyst 2900 series switch.

1900/2950 Firmware Upgrade
6.2.9 This page will explain the purpose of IOS and firmware upgrades and how they are performed.
IOS and firmware images are periodically released with bugs fixes, new features, and performance improvements. If the network can be made more secure, or can operate more efficiently with a new version of the IOS, then the IOS should be upgraded. 
To upgrade the IOS, download a copy of the new image to a local server from the Cisco Connection Online (CCO) Software Center.
The Lab Activities will show students how to upgrade the firmware of a switch.
This page concludes this lesson. The next page will summarize the main points from this module.

Executing adds, moves, and changes / Managing switch operating system file

Executing adds, moves, and changes 
6.2.6 The following are parameters that should be configured on a new switch that is added to a network: 

• Switch name
• IP address for the switch in the management VLAN
• A default gateway
• Line passwords

When a host is moved from one port or switch to another, configurations that can cause unexpected behavior should be removed. The switch can then be reconfigured to reflect the changes. 

The Lab Activities will teach students how to add, move, and change MAC addresses on a switch.








Managing switch operating system file
6.2.7 Network administrators should document and maintain the operational configuration files for network devices. The most current running-configuration file should be backed up on a server or disk. This is not only essential documentation, but is very useful if a configuration needs to be restored. 
The IOS should also be backed up to a local server. The IOS can then be reloaded to flash memory if needed.

The Lab Activities will show students how to create, verify, back up, and then restore a basic switch configuration.