Static and Dynamic Route Comparison

Routers can forward packets over static routes or dynamic routes based on the router configuration. The two ways to tell the router where to forward packets to destination networks that are not directly connected are as follows:

■ Static route: The router learns routes when an administrator manually configures the static route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update. Static routes are user-defined routes that specify the path that packets take when moving between a source and a destination. These administrator-defined routes allow very precise control over the

routing behavior of the IP internetwork.

■ Dynamic route: The router dynamically learns routes after an administrator configures a routing protocol that helps determine routes. Unlike the situation with static routes, after the network administrator enables dynamic routing, the routing process automatically updates route knowledge whenever new topology information is received. The router learns and maintains routes to the remote destinations by exchanging routing updates with other routers in the internetwork.

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Cisco IOS Software Features and Functions

Cisco IOS Software is the industry-leading and is the most widely deployed network system software. This topic describes the features and functions of Cisco IOS Software. The Cisco IOS Software platform is implemented on most Cisco hardware platforms, including switches, routers, and similar Cisco IOS–based network devices. It is the embedded software architecture in all Cisco devices and is also the operating system of Cisco Catalyst switches.

Cisco IOS Software enables the following network services in Cisco products:

■ Features to carry the chosen network protocols and functions.

■ Connectivity enables high-speed traffic between devices.

■ Security controls access and prohibit unauthorized network use.

■ Scalability adds interfaces and capability as needed for network growth.

■ Reliability ensures dependable access to networked resources.

The Cisco IOS Software command-line interface (CLI) is accessed through a console connection, a modem connection, or a Telnet session. Regardless of which connection method is used, access to the Cisco IOS software CLI is generally referred to as an EXEC session.

SUMMARY of Cisco IOS

■ 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.

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Layer 1/2/3 Devices and their Function

Layer 1 - Devices and Their Functions

Layer 1 defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Some common examples are Ethernet segments and serial links like Frame Relay and T1.

Repeaters that provide signal amplification are also considered Layer 1 devices.

The physical interface on the NIC can also be considered part of Layer 1.

Layer 2 Devices and Their Functions

Layer 2 defines how data is formatted for transmission and how access to the physical media is controlled. These devices also provide an interface between the Layer 2 device and the physical media. Some common examples are a NIC installed in a host, bridge, or switch.

Layer 3 Devices and Their Functions

The network layer provides connectivity and path selection between two host systems that might be located on geographically separated networks. In the case of a host, this is the path between the data link layer and the upper layers of the NOS. In the case of a router, it is the actual path across the network.

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Domain Name System

DNS is a mechanism for converting symbolic names into IP addresses. The DNS application frees users of IP networks from the burden of having to remember IP addresses. Without this freedom, the Internet would

not be as popular or as usable as it is. The DNS address is a server that provides the DNS services. The address is typically assigned during the DCHP address assignment or can be assigned manually.

To determine the actual address of the device, the command ipconfig can be used from the command line to display all current TCP/IP network configuration values and refresh DHCP and DNS settings. Used without parameters, ipconfig displays the IP address, subnet mask, and default gateway for all adapters. Figure 1-40 shows an example of an IPCONFIG output.

IPCONFIG Output

You can run ipconfig with various flags to determine exactly what output should be displayed. The syntax flags are as follows:

ipconfig [/all] [/renew [Adapter]] [/release [Adapter]] [/flushdns] [/displaydns] [/registerdns] [/showclassid Adapter] [/setclassid Adapter [ClassID]]

The parameters are as follows:

■ /all: Displays the full TCP/IP configuration for all adapters. Without this parameter, ipconfig displays only the IP address, subnet mask, and default gateway values for each adapter. Adapters can represent physical interfaces, such as installed network adapters, or logical interfaces, such as dialup connections.

■ /renew [Adapter]: Renews DHCP configuration for all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter is available only on computers with adapters that are configured to obtain an IP address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

■ /release [Adapter]: Sends a DHCPRELEASE message to the DHCP server to release the current DHCP configuration and discard the IP address configuration for either all adapters (if an adapter is not specified) or for a specific adapter if the Adapter parameter is included. This parameter disables TCP/IP for adapters configured to obtain an IP address automatically. To specify an adapter name, type the adapter name that appears when you use ipconfig without parameters.

■ /flushdns: Flushes and resets the contents of the DNS client resolver cache. During DNS troubleshooting, you can use this procedure to discard negative cache entries from the cache, as well as any other entries that have been added dynamically.

■ /displaydns: Displays the contents of the DNS client resolver cache, which includes both entries preloaded from the local hosts file and any recently obtained resource records for name queries resolved by the computer. The DNS client service uses this information to resolve frequently queried names quickly, before querying its configured DNS servers.

■ /registerdns: Initiates manual dynamic registration for the DNS names and IP addresses that are configured at a computer. You can use this parameter to troubleshoot a failed DNS name registration or resolve a dynamic update problem between a client and the DNS server without rebooting the client computer. The DNS settings in the advanced properties of the TCP/IP protocol determine which names are registered in DNS.

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OSI - Open System Interconnect Reference Model

The OSI reference model is the primary model for network communications. The early development of LANs, MANs, and WANs was chaotic in many ways. The OSI reference model, released in 1984, was the descriptive scheme that the ISO created. It provided vendors with a set of standards that ensured greater compatibility and
interoperability between the various types of network technologies produced by companies around the world. Although other models exist, most network vendors today relate their products to the OSI reference model, especially when they want to educate customers on the use of their products. The OSI model is considered the best tool available for teaching people about sending and receiving data on a network.

The OSI reference model has seven layers each illustrating a particular network function.

Each OSI layer contains a set of functions performed by programs to enable data to travel from a source to a destination on a network. The following sections provide brief descriptions of each layer in the OSI reference model.
Layer 7: The Application Layer
The application layer is the OSI layer that is closest to the user. This layer provides network services to the user’s applications. It differs from the other layers in that it does not provide services to any other OSI layer, but only to applications outside the OSI reference model. The application layer establishes the availability of intended communication partners and synchronizes and establishes agreement on procedures for error recovery and control of data integrity.
Layer 6: The Presentation Layer
The presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format.

Layer 5: The Session Layer
The session layer establishes, manages, and terminates sessions between two communicating hosts. It provides its services to the presentation layer. The session layer also synchronizes dialogue between the presentation layers of the two hosts and manages their data exchange. For example, web servers have many users, so many communication processes are open at a given time. Therefore, keeping track of which user communicates on which path is important. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems.
Layer 4: The Transport Layer
The transport layer segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system. For example, business users in large corporations often transfer large files from field locations to a corporate site. Reliable delivery of the files is important, so the transport layer breaks down large files into smaller segments that are less likely to incur transmission problems. The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data-transport issues. The transport layer attempts to provide a data-transport service that shields the upper layers from transport implementation details. Specifically, issues such as reliability of transport between two hosts are the concern of the transport layer. In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. Transport error detection and recovery and information flow control provide reliable
service.
Layer 3: The Network Layer
The network layer provides connectivity and path selection between two host systems that might be located on geographically separated networks. The growth of the Internet has increased the number of users accessing information from sites around the world, and the network layer manages this connectivity.
Layer 2: The Data Link Layer
The data link layer defines how data is formatted for transmission and how access to the network is controlled. This layer is responsible for defining how devices on a common media communicate with one another, including addressing and control signaling between devices.

Layer 1: The Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

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