Summary This page summarizes the topics discussed in this module. Redundancy is defined as a duplication of components that allows continued functionality despite the failure of an individual component. In a network, redundancy means to have a backup method to connect all devices. Redundant topologies increase network reliability and decrease downtime caused by a single point of failure. A redundant switched topology may cause broadcast storms, multiple frame transmissions, and MAC address table instability problems. A broadcast storm is caused by multiple hosts that send and receive multiple broadcast messages. The result is that they continue to propagate broadcast traffic over and over until one of the switches is disconnected. During a broadcast storm, the network appears to be down or extremely slow. Multiple frame transmissions occur when a router receives multiple copies of a frame from multiple switches due to an unknown MAC address. These excessive transmissions cause the ...
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Spanning-tree recalculation 7.2.6 This page will describe the convergence of a spanning-tree network. A switched internetwork has converged when all the switch and bridge ports are in either the forwarding or blocking state. Forwarding ports send and receive data traffic and BPDUs. Blocking ports only receive BPDUs. When the network topology changes, switches and bridges recompute the spanning-tree and cause a disruption in network traffic. Convergence on a new spanning-tree topology that uses the IEEE 802.1d standard can take up to 50 seconds. This convergence is made up of the max-age of 20 seconds, plus the listening forward delay of 15 seconds, and the learning forward delay of 15 seconds. The Lab Activities will show students how to create and verify a basic switch configuration. The next page will introduce the Rapid Spanning-Tree Protocol. Rapid spanning-tree protocol 7.2.7 This page will describe the Rapid Span...
Selecting the root bridge 7.2.4 This page will explain how a root bridge is selected in an STP network. The first decision that all switches in the network make, is to identify the root bridge. The position of the root bridge in a network affects the traffic flow. When a switch is turned on, the spanning-tree algorithm is used to identify the root bridge. BPDUs are sent out with the bridge ID (BID). The BID consists of a bridge priority that defaults to 32768 and the switch MAC address. By default BPDUs are sent every two seconds. When a switch first starts up, it assumes it is the root switch and sends BPDUs that contain the switch MAC address in both the root and sender BID. These BPDUs are considered inferior because they are generated from the designated switch that has lost its link to the root bridge. The designated switch transmits the BPDUs with the information that it is the root bridge as well as the designated bridge. These BPDUs contain the switch MAC addr...
Spanning Tree Protocol 7.2.3 This page will teach students about the ports and devices that are found in an STP switched network. When the network has stabilized, it has converged and there is one spanning-tree per network. As a result, for every switched network the following elements exist: One root bridge per network One root port per non-root bridge One designated port per segment Unused, or non-designated ports Root ports and designated ports are used for forwarding (F) data traffic. Non-designated ports discard data traffic. These ports are called blocking (B) or discarding ports. The next page will discuss the root bridge.
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 mes...
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...
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...
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 broad...
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 exp...
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 recon...
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 p...
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 prev...
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 oper...
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 fla...