1 апреля 2010 г.

Конспект к экзамену CCNA - ICND1 Book Chapter 2 - Ethernet LANs

Выдержки из Chapter 2 книги  Cisco Pres"Interconnecting Cisco Network Devices, Part 1 (ICND1): CCNA Exam 640-802 and ICND1 Exam 640-822" для повторения перед экзаменом. 
Ethernet Segment Distance Limitations
Summary of Ethernet Local-Area Networks
The key points that were discussed in the previous sections are as follows:
·         A segment is a network connection made by a single unbroken network cable. Ethernet cables and segments can only span a limited physical distance, after which the transmissions become degraded.
·         A hub works like a multiport repeater and can effectively extend a network segment by receiving the incoming frames, amplifying the electrical signals, and transmitting these frames back out through all ports to all devices that are connected to segments on the same Ethernet hub.
·         If two or more stations connected to the same hub transmit at the same time, a collision results because of the half-duplex nature of the Ethernet 802.3 CSMA/CD specification.
·         The network segments that share the same bandwidth are called collision domains because when two or more devices on the same segment both communicate and send data at the same time, collisions can occur within that shared segment.
·         It is possible to use other network devices, operating at Layer 2 (or above) of the OSI model, to divide network segments and reduce the number of devices that are competing or contending for bandwidth on any given segment and to provide better throughput for end users and devices on each separate segment.

Summary of Exploring the Packet Delivery Process
The key points that were discussed in the previous sections are as follows:
·         Operating systems use Layer 3 (IP) and Layer 2 (MAC) addresses to provide host-tohost communications.
·         Layer 2 switches forward frames based on entries in the port-mapping MAC address table.
·         Layer 2 switches learn the MAC addresses of devices that pass traffic through them to build the port-mapping table, and they learn MAC addresses by reading the source MAC address in a frame.
·         If the destination MAC address is unknown, meaning not in the MAC address table of the switch, or if the destination MAC is a broadcast, the frame is “flooded,” or sent out, all ports of the switch except for the source port.
·         Layer 2 switches do not change the traffic in the frame in any way.

Cisco CLI Error Messages
Command-Line Editing Keys
Cisco routers Primary types of memory
·         RAM: Stores routing tables and the fast-switching cache. RAM holds the current running configuration file, the currently loaded IOS, and so on.
·         NVRAM: Used for writable permanent storage of the startup configuration settings.
·         Flash: Provides permanent storage of the Cisco IOS Software image file, backup configurations, and any other files through memory cards.
·         ROM: Provides the POST routine and also provides a mini-IOS that can be used for troubleshooting and emergencies, such as when the stored IOS in flash is corrupted. The mini-IOS provided by ROM can also be for password recovery. ROM cannot be modified or copied to by device administrators.

Summary of Operating Cisco IOS Software
The key points that were discussed in the previous sections are as follows:
·         Cisco IOS Software is embedded software architecture in all the Cisco IOS devices and is also the operating system of Catalyst switches. Its functions include carrying the chosen network protocols, connectivity, security, scalability, and reliability.
·         A switch or IOS device can be configured from a local terminal connected to the console (CON) port, from a remote terminal connected through a modem connection to the auxiliary (AUX) port, or through a Telnet (VTY) connection.
·         The CLI is used by network administrators to monitor and configure various Cisco IOS devices. The CLI also offers a help facility to aid network administrators with the verification and configuration of commands.
·         The CLI supports two EXEC modes: user EXEC mode and privileged EXEC mode. The privileged EXEC mode provides more functionality than the user EXEC mode, and privileged EXEC mode is also sometimes called enable mode.
·         Cisco IOS devices use Cisco IOS Software with extensive command-line input help facilities, including context-sensitive help.
·         The Cisco IOS CLI includes an enhanced editing mode that provides a set of editing key functions.
·         A Cisco IOS device's CLI provides a history or record of the commands that have been entered.

Switch LEDs

Port LED Modes Part I

Port LED Modes Part II

Summary of Starting a Switch
The key points that were discussed in the previous sections are as follows:
·         The startup of a Cisco IOS switch requires verifying the physical installation, powering up the switch, and viewing the Cisco IOS Software output on the console.
·         The Cisco IOS switches have several status LEDs that are generally lit in green when the switch is functioning normally but turn amber when there is a malfunction.
·         The Catalyst POST is executed only when the switch is powered up.
·         During initial startup, if POST test failures are detected, they are reported to the console. If POST completes successfully, the switch can be configured.
·         When starting any EXEC mode session on a Cisco IOS switch, either locally or remotely, you begin in user EXEC mode. To change modes, like moving into privileged EXEC enable mode, a password must be entered, assuming that one has been set for securing the “all-access” privileged enable mode.
·         The Cisco IOS switch CLI provides a help facility that is similar to the help facility of the router.
·         The Catalyst IOS switches can be configured using global and other configuration modes; these modes are similar to the various EXEC modes at the CLI on Cisco routers.
·         After logging in to a Catalyst IOS switch, the switch software and hardware status can be verified using the show version, show running-config, and show interfaces commands.

Summary of Understanding Switch Security
The key points that were discussed in the previous sections are as follows:
·         User and Privileged Passwords can be used to restrict access levels to users that have different access needs for the device.
·         The first level of security is physical.
·         The login banner can be used to display a message before the user is prompted for a username.
·         Port security can be used to limit a MAC address to a port.
·         Unused ports should be shut down.

Summary of Maximizing the Benefits of Switching
The key points that were discussed in the previous sections are as follows:
·         Switched LANs provide microsegmentation, which means that each device on a network segment is connected directly to a switch port and receives its own bandwidth. Each device does not have to contend for bandwidth with any other device on the network.
·         Half-duplex communication in an Ethernet LAN using hubs allows data transmission in one direction at a time only (either sending or receiving). The full-duplex communication provided by a switch allows both sending and receiving of data simultaneously.
·         Using a basic core/distribution/access layer three-tier hierarchy of Ethernet connectivity is usually the most effective way to provide speed where it will be most effective in a campus network, implementing Fast Ethernet and Gigabit Ethernet primarily in workgroup and backbone connections.
·         Switches provide the valuable feature of redundancy to ensure availability of the network, but redundancy can cause loops.
·         Loops result when multiple switches are connected through multiple physical paths to the same segment and each transmits the same data. The data frames circulate between the two or more paths without being removed from the network and can cause inaccurate data in the MAC address tables of each switch.
·         The solution to loops is STP, which manages the paths to given network segments. STP provides path redundancy in an Ethernet LAN while preventing the undesirable effects of switching loops in the network.
Highlighted Fields for Troubleshooting

Summary of Troubleshooting Switch Issues
The key points that were discussed in the previous sections are as follows:
·         Use the show interface command to troubleshoot the following:
·         Media issues
·         Duplex issues
·         Speed issues
·         Keep a copy of device configurations.
·         Protect the running-config file.

Chapter Summary
The key points that were discussed in this chapter are as follows:
·         Ethernet cables and segments can only span a limited physical distance, but there are devices, such as repeaters and hubs, that can be added to an Ethernet LAN to extend the length of LAN segments.
·         Bridges and switches divide a LAN into multiple segments. However, switches operate at much higher speeds and support more advanced functionality, performing three major functions in segmenting an Ethernet network: forwarding, filtering, and flooding.
·         There are a number of ways in which the performance benefits of switched Ethernet LANs can be enhanced, including microsegmentation and tiered connectivity hierarchies. However, there is a potential for intentional or unintentional physical loops that can be resolved by implementing Spanning Tree Protocol.
·         The Cisco IOS CLI is used to communicate the configuration settings and details that implement the network requirements of an organization.
·         The startup of a Catalyst switch requires verifying the physical installation, powering up the switch, and viewing the Cisco IOS Software output on the console.
·         The CLI is used to configure the device name and passwords and to enter device modes such as global and interface configuration mode.
·         Increase switch security by enabling password and port security.
·         Most port access problems can be verified by using the show interface command.

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