What did I learn in this module?

Devices in a Small Network

Small networks typically have a single WAN connection provided by DSL, cable, or an Ethernet connection. Small networks are managed by a local IT technician or by a contracted professional. Factors to consider when selecting network devices for a small network are cost, speed and types of ports/interfaces, expandability, and OS features and services. When implementing a network, create an IP addressing scheme and use it on end devices, servers and peripherals, and intermediary devices. Redundancy can be accomplished by installing duplicate equipment, but it can also be accomplished by supplying duplicate network links for critical areas. The routers and switches in a small network should be configured to support real-time traffic, such as voice and video, in an appropriate manner relative to other data traffic. In fact, a good network design will implement quality of service (QoS) to classify traffic carefully according to priority.

Small Network Applications and Protocols

There are two forms of software programs or processes that provide access to the network: network applications and application layer services. Some end-user applications implement application layer protocols and are able to communicate directly with the lower layers of the protocol stack. Email clients and web browsers are examples of this type of application. Other programs may need the assistance of application layer services to use network resources like file transfer or network print spooling. These are the programs that interface with the network and prepare the data for transfer. The two most common remote access solutions are Telnet and Secure Shell (SSH). SSH service is a secure alternative to Telnet. Network administrators must also support common network servers and their required related network protocols such as web server, email server, FTP server, DHCP server, and DNS server. Businesses today are increasingly using IP telephony and streaming media to communicate with customers and business partners. These are real-time applications. The network infrastructure must support VoIP, IP telephony, and other real-time applications.

Scale to Larger Networks

To scale a network, several elements are required: network documentation, device inventory, budget, and traffic analysis. Know the type of traffic that is crossing the network as well as the current traffic flow. Capture traffic during peak utilization times to get a good representation of the different traffic types and perform the capture on different network segments and devices as some traffic will be local to a particular segment. Network administrators must know how network use is changing. Usage details of employee computers can be captured in a ‘snapshot’ with such tools as the Windows Task Manager, Event Viewer, and Data Usage.

Verify Connectivity

The ping command is the most effective way to quickly test Layer 3 connectivity between a source and destination IP address. The command also displays various round-trip time statistics. The Cisco IOS offers an “extended” mode of the ping command which lets the user create special types of pings by adjusting parameters related to the command operation. Extended ping is entered in privileged EXEC mode by typing ping without a destination IP address. Traceroute can help locate Layer 3 problem areas in a network. A trace returns a list of hops as a packet is routed through a network. It is used to identify the point along the path where the problem can be found. In Windows, the command is tracert. In Cisco IOS the command is traceroute. There is also an extended traceroute command. It allows the administrator to adjust parameters related to the command operation. The output derived from network commands contributes data to the network baseline. One method for starting a baseline is to copy and paste the results from an executed ping, trace, or other relevant commands into a text file. These text files can be time stamped with the date and saved into an archive for later retrieval and comparison.

Host and IOS Commands

Network administrators view the IP addressing information (address, mask, router, and DNS) on a Windows host by issuing the ipconfig command. Other necessary commands are ipconfig /allipconfig /release and ipconfig /renew, and ipconfig /displaydns. Verifying IP settings by using the GUI on a Linux machine will differ depending on the Linux distribution (distro) and desktop interface. Necessary commands are ifconfig, and ip address. In the GUI of a Mac host, open Network Preferences > Advanced to get the IP addressing information. Other IP addressing commands for Mac are ifconfig, and networksetup -listallnetworkservices and networksetup -getinfo <network service>. The arp command is executed from the Windows, Linux, or Mac command prompt. The command lists all devices currently in the ARP cache of the host, which includes the IPv4 address, physical address, and the type of addressing (static/dynamic), for each device. The arp -a command displays the known IP address and MAC address binding. Common show commands are show running-configshow interfacesshow ip addressshow arpshow ip routeshow protocols, and show version. The show cdp neighbor command provides the following information about each CDP neighbor device: identifiers, address list, port identifier, capabilities list, and platform. The show cdp neighbors detail command will help determine if one of the CDP neighbors has an IP configuration error. The show ip interface brief command output displays all interfaces on the router, the IP address assigned to each interface, if any, and the operational status of the interface.

Troubleshooting Methodologies

Step 1. Identify the problem

Step 2. Establish a theory of probably causes.

Step 3. Test the theory to determine the cause.

Step 4. Establish a plan of action and implement the solution.

Step 5. Verify the solution and implement preventive measures.

Step 6. Document findings, actions, and outcomes.

A problem should be escalated when it requires a the decision of a manager, some specific expertise, or network access level unavailable to the troubleshooting technician. OS processes, protocols, mechanisms and events generate messages to communicate their status. The IOS debug command allows the administrator to display these messages in real-time for analysis. To display log messages on a terminal (virtual console), use the terminal monitor privileged EXEC command.

Troubleshooting Scenarios

There are two duplex communication modes: half-duplex and full-duplex. If one of the two connected devices is operating in full-duplex and the other is operating in half-duplex, a duplex mismatch occurs. While data communication will occur through a link with a duplex mismatch, link performance will be very poor.

Wrongly assigned IP addresses create a variety of issues, including IP address conflicts and routing problems. Two common causes of incorrect IPv4 assignment are manual assignment mistakes or DHCP-related issues. Most end devices are configured to rely on a DHCP server for automatic IPv4 address assignment. If the device is unable to communicate with the DHCP server, then the server cannot assign an IPv4 address for the specific network and the device will not be able to communicate.

The default gateway for an end device is the closest networking device that can forward traffic to other networks. If a device has an incorrect or nonexistent default gateway address, it will not be able to communicate with devices in remote networks. Because the default gateway is the path to remote networks, its address must belong to the same network as the end device.

DNS failures often lead the user to conclude that the network is down. If a user types in a domain name such as www.cisco.com in a web browser and the DNS server is unreachable, the name will not be translated into an IP address and the website will not display.

17.8.5 Module Quiz – Build a Small Network