Cisco® CCENT Exam Cram Notes : Diagnostic Commands


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5. Infrastructure Services & Maintenance

5. Diagnostic Commands

Ping: The ping command is the most frequently used tool for troubleshooting the accessibility of network devices. It uses a series of Internet Control Message Protocol (ICMP) Echo messages to determine:

  • Whether a remote host is active or inactive.
  • The round-trip delay in communicating with the host.
  • Packet loss.

The ping command first sends an echo request packet to the address given in the ping command, then waits for a reply. The ping is successful only if the destination is able to provide an "echo reply" back to the source within a predetermined time called a timeout. The default value of this timeout is two seconds on Cisco routers.

ping - sends an ICMP echo message.

The syntax for ping command is : "ping <destination_address>"

The following are possible responses to "ping" command:

Ping Response Explanation
U destination Unreachable
C Congestion experienced 
! Successful receipt of echo reply
. Time out
? Packet type unknown
& Packet time to live exceeded

Ping sends ECMP echo. Ping can be used with almost any type of Network layer protocols including IPX, IP, VINES, AppleTalk etc.

Traceroute: The traceroute command is used to discover the route that a packet takes when traveling to it's destination. When you issue the command on a device (for example, a router or a PC), it sends out a sequence of UDP datagrams to an invalid port address at the remote host.

Three datagrams are sent, each with a Time-To-Live (TTL) field value set to one. The TTL value of 1 causes the datagram to "timeout" as soon as it hits the first router in the path; this router then responds with an ICMP Time Exceeded Message (TEM) indicating that the datagram has expired.

Telnet :Telnet command works at Layer 7 of your TCP/IP stack. If you are able to Telnet to your remote router, you can be sure that the TCP/IP stack is properly installed. TCP/IP is the protocol used when you are Telnetting to a remote host. HTTP is used for accessing the World Wide Web services.

Another three UDP messages are now sent, each with the TTL value set to 2, which causes the second router to return ICMP TEMs. This process continues until the packets actually reach the other destination. Since these datagrams are trying to access an invalid port at the destination host, ICMP Port Unreachable Messages are returned, indicating an unreachable port; this event signals the Traceroute program that it is finished.

The purpose behind this is to record the source of each ICMP Time Exceeded Message to provide a trace of the path the packet took to reach the destination.

Internet Control Message Protocol (ICMP) is an error reporting and diagnostic utility used by most network protocols including IOS routers and switches.

FTP: The File Transfer Protocol (FTP) is a standards based network protocol used to transfer computer files from one host to another host over a TCP/IP based network, such as the Internet. FTP is built on a client-server architecture uses port 21.

TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. Higher layer application protocols use TCP/IP suite of protocols to communicate over the the Internet. These include the World Wide Web's Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet (Telnet) which lets you logon to remote computers, and the Simple Mail Transfer Protocol (SMTP).

SNMP: Simple Network Management Protocol (SNMP) is an application-layer protocol for exchanging management information between network devices. It is a part of Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite.

UDP: UDP (User Datagram Protocol) is an alternative communications protocol to Transmission Control Protocol (TCP) used primarily for establishing low-latency and loss tolerating connections between applications on the Internet. Both UDP and TCP run on top of the Internet Protocol (IP) and are sometimes referred to as UDP/IP or TCP/IP

ICMP: Internet Control Message Protocol (ICMP) is an error reporting and diagnostic utility used by most network protocols including IOS routers and switches. It is also part of TCP/IP protocol suite of network protocols. ICMP (Internet Message Control Protocol) messages are used for basic error reporting between host to host, or host to gateway. It is not used for error reporting between Gateways. ICMP messages are encapsulated using the IP protocol. For example, the command "ping" uses ICMP protocol. In the OSI Reference model, ICMPs are generally considered part of the IP layer.

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