IP Troubleshooting Tips

IP Troubleshooting Tips

One important step in troubleshooting network communications is to verify the IP address, subnet mask, and default gateway settings of each host. Keep in mind the following as you troubleshoot IP:

• All computers must be assigned a unique IP address.
• Hosts on the same physical network should have IP addresses in the same address range.
• The subnet mask value for all computers on the same physical network must be the same.
• Configure the default gateway value to enable internetwork communication.
• The default gateway address must be on the same subnet as the host's IP address.
• You do not need to configure an IP address on a switch for frames to be switched through the switch. To ping to and from a switch or to remotely manage the switch, configure an IP address on the switch.

Listed below are several common symptoms and things to try to correct communication problems.

Problem	Symptoms	Solution
A single host cannot communicate with any other host.	Ping to any other host fails.	Because the problem exists with only one host, troubleshoot the configuration of the host with the problem.
A single host can communicate with all hosts on the same network, but can't communicate with any host on any other network.	Ping to hosts on the same network succeed, ping to hosts on other networks fails.	Verify the default gateway setting of the host with the problem. Because only a single host has the problem, you should be able to assume that the default gateway device is functioning correctly.
	Traceroute on the host times out with only a single entry.
All hosts can communicate within the same network, but cannot communicate with any host outside of the local network.	Ping to hosts on the same network succeed, ping to hosts on other networks fails.	Check for the following: If hosts have an IP address on the 169.254.0.0/16, then APIPA was used to assign the IP address and the default gateway value will be missing. Verify that the DHCP server is up. If DHCP is used to assign IP information to hosts, verify the default gateway setting delivered by the DHCP server. Verify that the default gateway device is up, has a valid connection to all networks, and has routing table information to reach destination networks.
	Traceroute on the host times out with only a single entry.
All hosts cannot communicate with hosts on a specific outside network. Communication with other networks is fine.	Ping to the remote network fails, traceroute on the host times out with only a single entry.	Add a route to the routing table, or configure the gateway of last resort (default route) on the router. The gateway of last resort is also known as the default gateway for the router.
	The routing table on the router does not show the destination network, or the gateway of last resort is not set.
	The routing table has a route to the destination network. Traceroute on the router times out.	Troubleshoot other routers in the path to the destination network. Use traceroute to identify the last responding router and begin troubleshooting there.
All hosts cannot communicate with a specific remote host. Communication with other remote hosts in the same remote network is fine.	Ping to the remote host fails. Traceroute to the remote host indicates no response from the host.	Troubleshoot the configuration of the remote host.
	The routing table shows a route to the destination network (or the gateway of last resort is used).

Workstation TCP/IP Utilities

Workstation TCP/IP Utilities

In addition to using ping, tracert, and Telnet on a Windows workstation to test Network- and Application-layer connectivity, you can use the following utilities to verify the configuration of the workstation.

Utility	Description
Ipconfig	Ipconfig displays IP configuration information for network adapters including: IP address and mask Default gateway DNS and WINS server addresses IP address of the DHCP server used for configuration MAC address Use the ipconfig command as follows: Use ipconfig to view IP address, subnet mask, and default gateway configuration Use ipconfig /all to view detailed configuration information. Use ipconfig /release to release the IP configuration information obtained from the DHCP server. Use ipconfig /renew to request new IP configuration information from the DHCP server.
Arp	The ARP cache keeps a mapping of IP address to MAC addresses. If the IP address or MAC address changes, the value in the cache might be out of date. Use the arp -a to list a host's ARP cache. Use arp -d to clear the ARP cache. Use arp -d * to remove all dynamic ARP entries from the ARP list. Note: Switches used with arp are case-sensitive. Arp -a is not the same thing as arp -A.
Nslookup	Nslookup resolves (looks up) the IP address of a host name. Displays other name resolution-related information such as the DNS server used for the lookup request.

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.

ICMP Messages

ICMP Messages

The Internet Control Message Protocol (ICMP) is a special-purpose message mechanism added to the TCP/IP suite that lets computers and routers in an internetwork report errors or provide information about unexpected circumstances. Remember that IP is a connectionless protocol and as such, contains no procedures that help to monitor successful packet delivery or test connectivity. Hosts use ICMP to send error messages to other hosts.

ICMP messages include the following types:

Message	Characteristics
Echo	The ICMP echo message is used to discover hosts and networks, and to verify that they are reachable. The ping utility is a popular utility that uses ICMP echo messages.
Destination unreachable	The destination unreachable message is sent if a packet cannot reach its destination for a variety of reasons. It might indicate the host is unavailable, or that there were problems detected in the packet header.
Time exceeded	The time exceeded message is sent when the packet's time-to-live (TTL) counter has expired.
Redirect	The redirect message is sent from a router to the sending device to indicate that a different route should be chosen for the packet. The redirect message can be sent if a better route is in the router's table, or if the selected route is unavailable or congested.
Source quench	The source quench message is sent by a receiving device to indicate that the flow of packets is too fast. When a sending device receives a source quench message, it slows its rate of transmission.
Router discovery	The router discovery message is a special broadcast message sent by hosts to discover the routers on a network. Routers respond to the message indicating their presence. They do not exchange routing information, but simply announce their availability.