Using The Ping Command
Using the ping command is one an effective way to test network connection. 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.
Using ping in a Testing Sequence
Firstly, start by using the router IOS ping command in a planned sequence of steps to establish valid connections, starting with the individual device and then all the way to the LAN and, finally, to remote networks.
By using the ping command in this ordered sequence, problems can be put out-of-the-way. The ping command sometimes does not always pinpoint the nature of the problem, but it can help to identify the source of the problem, this is considered to be the 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. There are additional tools that can provide more information than ping, such as Telnet or Trace, which we will look at in detail later.
IOS Ping Indicators
A ping from the IOS will yield to one of several indications for each ICMP echo that was sent. These indicators are:
! - Exclamation mark
. - Period and
U
- ! - 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 connectivity problem occurred somewhere along the path. It also may indicate 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 and responded with an ICMP unreachable message.
Pinging the Loopback
As a first step in the testing sequence, the ping command is used to verify the internal IP configuration on the local host. This can be accomplished by using the ping command on a reserved address called the loopback -127.0.0.1-. Pinging the loopback helps to verify 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 into a command line.
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 shows that four test packets were sent - each 32 bytes in size - 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.
Verifying Interface Connection
The IOS provides commands to verify the operation of router and switch interfaces. You can use the following command Verify Router Interfaces:
The show ip interface brief command provides a summary of all interface configuration information on the router; it displays the IP addresses that are assigned to the interface and other operational status of the interface.
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.1 YES manual up up
FastEthernet0/1 172.17.1.1 YES manual up up
Serial0/0/0 unassigned YES manual administratively down down
Serial0/0/1 unassigned YES manual administratively down down
Vlan1 unassigned YES manual administratively down down
Router#
Looking at the line for the FastEthernet 0/0 interface, we see that the IP address is 192.168.1.1. Looking at the last two columns, we can see the Layer 1 and Layer 2 status of the interface. The up in the Status column shows that this interface is operational at Layer 1. The up in the Protocol column indicates that the Layer 2 protocol is operational also the fastEthernet 0/1 with IP address 172.17.1.1, in this case.
In the same example above, notice that the Serial 0/0/0 and Serial0/0/1 interfaces have not been enabled and no IP address assigned. This is indicated by administratively down in the Status column. This interface can be enabled with the no shutdown command.
Testing Router Connectivity
We can use Ping and Traceroute to verify router connectivity, at the layer 3. You can use these commands to ping a host in a local LAN and place a trace to a remote host across the WAN.
e.g.
Router#ping 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 15/15/16 ms
Router#traceroute 192.168.1.1
Type escape sequence to abort.
Tracing the route to 192.168.1.1
1 192.168.1.1 16 msec 16 msec 16 msec
The above result shows a successful connection to the gateway.
Testing NICs
The next step in the testing sequence is to verify that the Network Interface Card- NIC- address is bound to the IPv4 address and that the NIC is ready to transmit signals across the media.
The IPv4 address assigned to a NIC in this case is 10.0.0.6.
To verify the IPv4 address, use the following steps:
Use the following command:
C:>ping 10.0.0.6
A successful reply would resemble:
Reply from 10.0.0.6: bytes=32 time<1ms TTL=128
Reply from 10.0.0.6: bytes=32 time<1ms TTL=128
Reply from 10.0.0.6: bytes=32 time<1ms TTL=128
Reply from 10.0.0.6: bytes=32 time<1ms TTL=128
Ping statistics for 10.0.0.6:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
This test verifies that the NIC driver and most of the NIC hardware are working properly. It also verifies that the IP address is properly bound to the NIC, without actually putting a signal on the media.
If this test fails, it is likely that there are issues with the NIC hardware and software driver that may require reinstallation of either or both. This procedure is dependent on the type of host and its operating system
