Discuss 4 software or hardware problems that will render two hosts unable to use

Question

Discuss 4 software or hardware problems that will render two hosts unable to use the transmission control protocol/Internet protocol (TCP/IP) protocol to communicate.

Discussion Board Assignment Guidelines

Use the course materials and the Web to research network component troubleshooting.

Answer the following question:What are 4 problems that will break TCP/IP communication between two hosts? Briefly explain.

The selected problems can pertain to hardware issues, software issues, or a combination of both.

What network troubleshooting tools would you recommend for identifying the problems?

How would you use them to address the issues listed above?

What advantages and disadvantages does each tool present?

Explanation / Answer

1Q.What are 4 problems that will break TCP/IP communication between two hosts? Briefly explain. The selected problems can pertain to hardware issues, software issues, or a combination of both.

Ans:

TCP/IP connectivity problem, it is necessary to identify a single pair of source and destination devices that are exhibiting the connectivity problem. When you've selected the two devices, test to make sure that the problem is actually occurring between these two devices.

Possible problems include these:

•Physical layer issue somewhere along the path

•First-hop Layer 3 connectivity issue, local LAN segment

•Layer 3 IP connectivity issue somewhere along the packet's path

•Name resolution issue

4 problems that will break TCP/IP communication between two hosts.

Those are

2.     Troubleshooting Physical Connectivity Problems

3.     Troubleshooting IP Connectivity and Routing Problems

4.     Troubleshooting Upper-Layer Problems

1.Troubleshooting Local Connectivity Problems

This section describes how to troubleshoot local connectivity problems on LAN segments such as Ethernet or Token Ring. Going through the methodology in this chapter with help determine and resolve problems moving packets on the local LAN segment or to the next-hop router. If the problem is determined to be past the local LAN segment, then you will be referred to the section "Troubleshooting IP Connectivity and Routing Problems," later in this chapter. If the source device is connected via a modem, then you should consult Chapter 16, "Troubleshooting Dialup Connections."

Possible problems include these:

•Configuration problem

•DHCP or BOOTP issue

•Physical layer issue

•Duplicate IP address

   Check for Configuration Problems

To begin troubleshooting, display and examine the IP configuration of the source device. The method to determine this information varies greatly from platform to platform. If you are unsure of how to display this information, consult the manual for the device or operating system. Refer to the following examples:

•On a Cisco router, use show ip interface and show running-config.

•On Windows 95 or 98, use winipcfg.exe.

•On Windows 2000 or NT, use ipconfig.exe.

•On a UNIX platform, use ifconfig.

Check for Local Connectivity

If the destination is on the same subnet as the source, try pinging the destination by IP address. If the destination is on a different subnet, then try pinging the default gateway or appropriate next hop obtained from the routing table. If the ping fails, double-check the configuration of the next-hop router to see if the subnet and mask match the source's configuration.

Ruling Out Duplicate IP Addresses

To rule out a duplicate IP address, you can disconnect the suspect device from the LAN or shut down the suspect interface and then try pinging the device from another device on that same LAN segment. If the ping is successful, then there is another device on that LAN segment using the IP address. You will be able to determine the MAC address of the conflicting device by looking at the ARP table on the device that issued the ping.

2.Troubleshooting Physical Connectivity Problems

This section describes how to troubleshoot Layer 1 and 2 physical connectivity issues on LANs such as Ethernet or Token Ring. For troubleshooting information on dialup links or WAN connections, consult the chapters in Part IV, "Troubleshooting Serial Lines and WAN Connections."

Even though it may seem logical to first troubleshoot at the physical layer, problems can generally be found more quickly by first troubleshooting at Layer 3 and then working backward when a physical problem is found or suspected.

Possible problems include these:

•Configuration is incorrect.

•Cable is faulty or improperly connected.

•Wiring closet cross-connect is faulty or improperly connected.

•Hardware (interface or port) is faulty.

•Interface has too much traffic.

Rule Out a Configuration Problem

Check to make sure that all cables are connected to the appropriate ports. Make sure that all cross-connects are properly patched to the correct location using the appropriate cable and method. Verify that all switch or hub ports are set in the correct VLAN or collision domain and have appropriate options set for spanning tree and other considerations.

Check Cable Connections

Verify that the proper cable is being used. If this is a direct connection between two end systems (for example, a PC and a router) or between two switches, a special crossover cable may be required.

Check the Configuration

Verify that the interface on the device is configured properly and is not shut down. If the device is connected to a hub or switch, verify that the port on the hub or switch is configured properly and is not shut down. Check both speed and duplex.

Check the Network Interface

Most interfaces or NICs will have indicator lights that show whether there is a valid connection; often this light is called the link light. The interface may also have lights to indicate whether traffic is being sent (TX) or received (RX). If the interface has indicator lights that do not show a valid connection, power off the device and reseat the interface card.

3.Troubleshooting IP Connectivity and Routing Problems

When troubleshooting IP connectivity problems across large networks, it always helps to have a network diagram handy so that you can understand the path that the traffic should take and compare it to the path that it is actually taking.

When IP packets are routed across a network, there is the potential for problems at every hop between the source and the destination, so test connectivity at each hop to determine where it is broken is the logical troubleshooting methodology.

The following could be wrong:

•A router may not have a route to the source or destination.

•The network might have a routing loop or other routing protocol-related problem.

•A physical connectivity problem might have occurred.

•A resource problem on one router might be prohibiting proper router operation. This could possibly be caused by lack of memory, lack of buffers, or lack of CPU.

•A configuration problem might have occurred on a router.

•A software problem might have occurred on a router.

•A packet filter or firewall might be preventing traffic from passing for an IP address or protocol.

•An MTU mismatch problem might have occurred.

4.Troubleshooting Upper-Layer Problems

Even though there may be IP connectivity between a source and a destination, problems may still exist for a specific upper-layer protocols such as FTP, HTTP, or Telnet. These protocols ride on top of the basic IP transport but are subject to protocol-specific problems relating to packet filters and firewalls. It is possible that everything except mail will work between a given source and destination. Before troubleshooting at this level, it is important to first establish whether IP connectivity exists between the source and the destination. If IP connectivity exists, then the issue must be at the application layer.

The following could go wrong:

•A packet filter/firewall issue might have arisen for the specific protocol, data connection, or return traffic.

•The specific service could be down on the server.

•An authentication problem might have occurred on the server for the source or source network.

•There could be a version mismatch or incompatibility with the client and server software.

2Q .What network troubleshooting tools would you recommend for identifying the problems?

Ans:

Tools for Troubleshooting IP Problems

The tools ping and traceroute, both in the TCP/IP protocol suite, will greatly assist in troubleshooting IP connectivity. Most operating systems and IP implementations come with these tools installed by default. On some UNIX platforms, however, you may need to download and install a traceroute package.

Cisco routers provide a basic method of viewing IP traffic switched through the router called packet debugging. Packet debugging enables a user to determine whether traffic is travelling along an expected path in the network or whether there are errors in a particular TCP stream. Although in some cases packet debugging can eliminate the need for a packet analyzer, it should not be considered a replacement for this important tool.

Packet debugging can be very intrusive—in some cases, it can cause a router to become inoperable until physically reset. In other instances, packets that are present on the network and switched through the router may not be reported by packet debugging. Thus, a firm conclusion cannot be drawn that a packet was not sent solely from the output of packet debugging; a network analyzer must be used to accurately make this assessment. Packet debugging should be used with extreme caution by only advanced operators because it can cause the router to lock up and stop routing traffic, if not used carefully. The risks of using packet debugging may be compounded by the necessity of disabling fast switching for packet debugging to be effective. As a general rule, packet debugging should not be used on a production router unless you have physical access to the router and are willing to risk it going down.

1.ping

The ping tool uses the IP ICMP echo request and echo reply messages to test reachability to a remote system. In its simplest form, ping simply confirms that an IP packet is capable of getting to and getting back from a destination IP address (Figure 7-7). This tool generally returns two pieces of information: whether the source can reach the destination (and, by inference, vice versa), and the round-trip time (RTT, typically in milliseconds). The RTT returned by ping should be used only as a comparative reference because it can depend greatly on the software implementation and hardware of the system on which ping is run. If ping fails or returns an unusual RTT, traceroute can be used to help narrow down the problem. It is also possible to vary the size of the ICMP echo payload to test problems related to maximum transmission unit (MTU).

Table 7-2 Cisco ping Return Codes

Code

Meaning

Possible Cause(s)

!

Each exclamation point indicates receipt of an ICMP echo reply.

The ping completed successfully.

.

Each period indicates that the network server timed out while waiting for a reply.

This message can indicate many problems:

•ping was blocked by an access list or firewall.

•A router along the path did not have a route to the destination and did not send an ICMP destination unreachable message.

•A physical connectivity problem occurred somewhere along the path.

U

An ICMP unreachable message was received.

A router along the path did not have a route to the destination address.

C

An ICMP source quench message was received.

A device along the path—possibly the destination—may be receiving to much traffic; check input queues.

&

An ICMP time exceeded message was received.

A routing loop may have occurred.

2.traceroute

The traceroute utility sends out either ICMP echo request (Windows) or UDP (most implementations) messages with gradually increasing IP TTL values to probe the path by which a packet traverses the network (see Example 7-3). The first packet with the TTL set to 1 will be discarded by the first hop, and the first hop will send back an ICMP TTL exceeded message sourced from its IP address facing the source of the packet. When the machine running the traceroute receives the ICMP TTL exceeded message, it can determine the hop via the source IP address. This continues until the destination is reached. The destination will return either an ICMP echo reply (Windows) or a ICMP port unreachable, indicating that the destination had been reached. Cisco's implementation of traceroute sends out three packets at each TTL value, allowing traceroute to report routers that have multiple equal-cost paths to the destination.

Traceroute can return useful information about TCP/IP connectivity across your network. Table 7-3 shows some of the codes that can be returned by the Cisco traceroute utility, along with their possible cause.

Cisco traceroute Return Codes

Code

Meaning

Possible Cause(s)

nnmsec

This gives, for each node, the round-trip time (in milliseconds) for the specified number of probes.

This is normal.

*

The probe timed out.

A device along the path either did not receive the probe or did not reply with an ICMP "packet life exceeded" message.

A

Administratively prohibited.

A device along the path, such as a firewall or router, may be blocking the probe and possibly other or all traffic; check access lists.

Q

Source quench.

A device along the path may be receiving to much traffic; check input queues.

H

An ICMP unreachable message has been received.

A routing loop may have occurred.

3.Packet Debugging:

The debug ip packet command should always be run with an access list to restrict the traffic that it will display. Failure to do so will almost certainly lock up the router. Even with an access list, there is always a possibility that packet debugging will lock up the router. Do not run this utility on a router that cannot be brought down or physically reset.

3Q.How would you use them to address the issues listed above?

Ans:

It is possible for IP connectivity to work but for DNS name resolution to fail. To troubleshoot this situation, use one of the following methods to determine whether DNS is resolving the name of the destination:

•Ping the destination by name, and look for an error message indicating that the name could not be resolved.

•If you are working on a UNIX machine, use nslookup <fully-qualified domain name> to perform a DNS lookup on the destination. If it is successful, the host's address should be displayed:

unix% nslookup www.somedomain.com

Server: localhost

Address: 127.0.0.1

Non-authoritative answer:

Name:    www.somedomain.com

Address: 10.1.1.1

If nslookup fails, the following output resembles the following sample output:

unix% nslookup www.somedomain.com

Server: localhost

Address: 127.0.0.1

*** localhost can't find www.notvalid.com: Non-existent host/domain

If DNS correctly resolves the host's name, go to the section "Narrowing Down the Problem Domain," earlier in this chapter, to start troubleshooting again. Otherwise, continue troubleshooting as follows:

1. Determine which name server you are using; this can be found in different places on each operating system, so if you are unsure of how to find it, consult the device's manual. For examples:

–On a Cisco router, type show run and look for the name-server.

–On Windows 95 or 98, use winipcfg.exe.

–On Windows 2000 or NT, use ipconfig.exe.

–On a UNIX platform, type cat /etc/resolv.conf at a command prompt.

2. Verify that you can ping the name server using its IP address. If the ping fails, go to the section "Narrowing Down the Problem Domain," earlier in this chapter, to troubleshoot connectivity between the client and the name server.

3. Verify that you can resolve names within your domain. (For example, if your host is Host1.test.com, you should be able to resolve the names of other hosts in the test.com domain, such as host2.test.com.)

4. Verify that you can resolve one or more domain names outside your domain.

If you cannot resolve names from all domains except that of the destination, there might be a problem with the DNS for the destination host. Contact the administrator of the destination device.

If you cannot resolve names within your domain or a large number of external domains, contact your DNS administrator because there may be a problem with the local DNS (or your host could be using the wrong domain server).

4Q.What advantages and disadvantages does each tool present?

Ans:

1.Ping:

                Advantage:

          Disadvantage

2. traceroute:

                Advantage:

          Disadvantage

3.Packet Debugging

                    The following example is applicable to Cisco 16xx, 25xx, 26xx, 36xx, 40xx, 45xx, 70xx, and 75xx series routers. Consult the Cisco TAC web page www.cisco.com/tac for instructions on using this command on other Cisco router platforms.

Advantage:

The debug ip packet command should always be run with an access list to restrict the traffic that it will display.

Disadvantage:

Failure to do so will almost certainly lock up the router. Even with an access list, there is always a possibility that packet debugging will lock up the router. Do not run this utility on a router that cannot be brought down or physically reset.

Table 7-2 Cisco ping Return Codes

Code

Meaning

Possible Cause(s)

!

Each exclamation point indicates receipt of an ICMP echo reply.

The ping completed successfully.

.

Each period indicates that the network server timed out while waiting for a reply.

This message can indicate many problems:

•ping was blocked by an access list or firewall.

•A router along the path did not have a route to the destination and did not send an ICMP destination unreachable message.

•A physical connectivity problem occurred somewhere along the path.

U

An ICMP unreachable message was received.

A router along the path did not have a route to the destination address.

C

An ICMP source quench message was received.

A device along the path—possibly the destination—may be receiving to much traffic; check input queues.

&

An ICMP time exceeded message was received.

A routing loop may have occurred.

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