After the network has been implemented, a network administrator must be able to test the network connectivity to ensure that it is operating appropriately. Additionally, it is a good idea for the network administrator to document the network
The Ping Command
Using the ping command is an effective way to test connectivity. The test is often referred to as testing the protocol stack, because the ping command moves from Layer 3 of the OSI model to Layer 2 and then Layer 1. Ping uses the ICMP protocol to check for connectivity.
The ping command will not always pinpoint the nature of a problem, but it can help to identify the source of the problem, an important first step in troubleshooting a network failure.
The ping command provides a method for checking the protocol stack and IPv4 address configuration on a host as well as testing connectivity to local or remote destination hosts, as shown in the figure. There are additional tools that can provide more information than ping, such as Telnet or Trace, which will be discussed in more detail later.
IOS Ping Indicators
A ping issued from the IOS will yield one of several indications for each ICMP echo that was sent. The most common indicators are:
- ! - indicates receipt of an ICMP echo reply message
- . - indicates a time expired while waiting for an ICMP echo reply message
- U - an ICMP unreachable message was received
The "!" (exclamation mark) indicates that the ping completed successfully and verifies Layer 3 connectivity.
The "." (period) can indicate problems in the communication. It may indicate that a connectivity problem occurred somewhere along the path. It may also indicate that a router along the path did not have a route to the destination and did not send an ICMP destination unreachable message. It also may indicate that ping was blocked by device security.
The "U" indicates that a router along the path did not have a route to the destination address or that the ping request was blocked and responded with an ICMP unreachable message.
Testing the Loopback
The ping command is used to verify the internal IP configuration on the local host. Recall that this test is accomplished by using the ping command on a reserved address called the loopback (127.0.0.1). This verifies the proper operation of the protocol stack from the network layer to the physical layer - and back - without actually putting a signal on the media.
Ping commands are entered at a command line.
Enter the ping loopback command with this syntax:
C:\> ping 127.0.0.1
The reply from this command would look something like this:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Ping statistics for 127.0.0.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
The result indicates that four 32 byte test packets were sent and were returned from host 127.0.0.1 in a time of less than 1 ms. TTL stands for Time-to-Live and defines the number of hops that the ping packet has remaining before it will be dropped.