TCP/IP Utilities

TCP/IP Utilities

The following table describes three utilities you can use to test network connectivity between devices. You can use these utilities on Windows workstations as well as Cisco devices.

Utility	Description
Ping	Ping sends an ICMP echo request/reply packet to a remote host. A response from the target device verifies that the host can communicate with the destination. Ping operates at the Network layer. A successful ping test verifies Network-layer connectivity between devices as well as the TCP/IP configuration of all devices in the path. Ping reports success or failure, together with round-trip statistics.
Traceroute	Traceroute uses ICMP echo request/reply packets together with the Time-to-Live (TTL) value in those packets to identify the path between two devices. Traceroute sends successive ICMP messages to a destination with increasing TTL values. For example, the first test pings the destination using a TTL of 1, the second pings with a TTL of 2, and so on. By default, traceroute sends three ping tests for each TTL value. Traceroute operates at the Network layer. Like ping, a successful test verifies Network-layer connectivity and TCP/IP configuration of devices in the path. Traceroute reports success or failure for each hop in the path, along with the IP address and hostname (if available) of each hop. Statistics for each hop are also reported. On a Windows workstation, use the tracert command to perform a traceroute test.
Telnet	Telnet is an application that establishes a remote session with a destination device. For example, you use Telnet from a workstation or a Cisco device to create a remote console session with another device. Telnet operates at the Application layer. A successful test verifies Application-layer connectivity. Because Telnet relies on lower-layer protocols, a successful Telnet session also verifies Network-layer and TCP/IP configuration. A successful Telnet test opens a remote connection to the target device. Note: A successful Telnet test means that ping and traceroute will also be successful. A failed Telnet test only indicates a failure at the Application layer or below. By itself, it does not tell you at which layer the problem exists.

Be aware of the following when working with these utilities on Cisco devices:

• When using ping, an exclamation mark indicates a successful ping, while a period indicates a failure.
• Both ping and traceroute include a standard or an extended mode.
  • Extended mode is available only in privileged EXEC mode.
  • Use extended mode to modify the number of tests performed or the timeout period.
  • Use extended mode to test non-IP protocols (such as AppleTalk or Novell IPX).
• Responses to each test within the traceroute command are as follows:
  • A time exceeded message indicates that a router has received the packet but the TTL has expired. For example, if the TTL is set to 3, the third router in the path responds with the time exceeded message.
  • A destination unreachable message indicates that the router in the path does not have a route to the destination network or device, or the destination device is down.
  • An asterisk ( * ) indicates that the timer has expired without a response.

  Note: The time exceeded and destination unreachable messages depend on the configuration of the intermediary and destination devices. Many devices are configured to not respond to ICMP messages, so you might see an asterisk even if the router in the path has received the packet.

• When using Telnet:
  • To suspend a Telnet session, press Ctrl + Shift + 6, then X.
  • To resume a Telnet session, use the resume command.
  • By default, debug information shows only on the console, not in the Telnet session window. Use the terminal monitor command to show debug information in a Telnet session.