Troubleshooting IP Routing

EXAM 70-293 OBJECTIVE 2, 2.1.2, 3, 3.4

Here, we will look at the two main tools you might use in troubleshooting IP routing and the common problems that occur with IP routing, which you will be expected to know how to deal with in the exam.

Note

There are entire books devoted to troubleshooting IP routing. Also, Microsoft’s online help system is fairly good at suggesting probable causes and solutions for many common routing problems.

Identifying Troubleshooting Tools

Your best troubleshooting tools are those tools you should be using on a daily basis for network management and monitoring. Windows Server 2003 ships with the Network Monitor tool (NETMON.exe), which is an excellent protocol analyzer that you can use to monitor your network. As discussed in Chapter 3, this tool captures and displays information about the IP packets moving in your network and can tell you about traffic patterns, broadcast rates, how the network is being used, what kinds of errors you might be experiencing, and many other aspects concerning the behavior of your network.

The Routing and Remote Access console is another excellent troubleshooting tool. Using this tool, you can show your network’s TCP/IP information, your IP routing table, the router’s RIP neighbors, its OSPF area, the LSDB, the router’s OSPF neighbors, and the OSPF virtual interface.

Other familiar tools that you can use for troubleshooting include PING, pathping, tracert, mrinfo, and netsh. Let’s take a look at how you can these tools to verify and troubleshoot your connections.

Configuring & Implementing...
Testing your TCP/IP Connections with PING

To use PING to test your TCP/IP connections, follow these steps:

1. Click Start | Run, type cmd, and press the Enter key to bring up the command prompt.

2. Using the ipconfig command, discussed in Chapter 3, determine the IP addresses of your computer and your default gateway.

3. Making sure that TCP/IP is installed and working on your local computer. Then type ping 127.0.0.1 at the command prompt and press the Enter key. You should receive a response in the command prompt window displaying four replies from the 127.0.0.1 loopback address. If not, you will need to reset the TCP/IP configuration on your machine.

4. If you received the proper replies, test the IP address of your local machine that you obtained from the ipconfig command by pinging it. If you receive the correct four replies, you know that your computer was added to the network correctly.

5. Ping the default gateway address to verify that it is up and running. This also lets you know if you are able to connect to a local host on your local network.

6. Ping the IP address or hostname of another remote host. You can ping a hostname by typing ping www.microsoft.com. This will let you know that you are able to communicate through a router.

Another useful troubleshooting tool is the pathping command. This command combines aspects of PING and tracert, and adds in some additional features that make it an excellent troubleshooting tool. This tool works by measuring the packet loss across each router between the source machine and the destination. This information can help you determine where your network reliability problems may be coming from. The syntax for the pathping command is as follows:

pathping [-n] [-h maximum_hops value] [-g host-list] [-p value]      [-q value] [-w value] final_destination 

Where:

• -n Tells pathping not to resolve addresses to host names.

• -h maximum_hops value Sets the maximum number of hops you want the command to search for the target. The default is 30 hops.

• -g host-list Provides a loose source route along the host list.

• -p period Sets the wait period in milliseconds between pings. The default is 250 milliseconds.

• -q num_queries Sets the number of queries per hop. The default is 100 queries.

• -w timeout Sets the time length in milliseconds for each reply before the command times out on that hop. The default is 3000 milliseconds.

• -T Tests the connectivity to each hop with Layer-2 priority tags.

• -R Tests to see if each hop is RSVP-aware.

• final_destination The host name or IP address of the network, domain, or machine that you are testing the route to.

The tool will first trace the route to the destination, and then analyze the traffic running through each hop. Keep in mind that one test is not sufficient to give you a good idea about what is going on. There is no specific number of lost packets that signify that a link is causing you problems. If the number is in double digits, though, you should probably examine that route carefully. To get a realistic picture of what is going on in your network, test a router over time and test in both peak and off-peak usage.

If you’re using multicast routing, another useful troubleshooting command is mrinfo. This command displays multicast router configuration information. The syntax is as follows:

mrinfo [-n] [-?] [-i address] [-t secs] [-r retries] destination

Where:

• -n Displays the IP addresses in numeric format.

• -? Prints usage information.

• -i Specifies the IP address of the local interface from which the query was sent.

• -r Specifies how many times an SNMP query is to be resent. The default value is 0.

• -t Specifies how long to wait for an IGMP neighbor query reply. The default is three seconds.

The mrinfo command displays the interfaces for both the multicast router and its neighbors on each interface. It also provides the names of the neighboring domains, the multicast routing metric, and the TTL.

Also, the netsh utility, discussed in the “Using netsh Commands” section earlier in this chapter, can display the configurations of protocols, filters, and routes. It also allows you to reconfigure interfaces. Don’t overlook this valuable tool as an option for troubleshooting IP routing.

Head of the Class…
Using Tracert to Test TCP/IP Connections

You can also use Tracert to test your TCP/IP connections. Just follow these steps:

1. Click Start | Run, type cmd, and press the Enter key to bring up the command prompt window.

2. At the prompt, type tracert target-ipaddress and press the Enter key. Replace target-ipaddress with the IP address of the remote network host you are attempting to connect with. This can also be a host name.

   The display will now include a list of the routers the packets have successfully crossed, along with the length of time the packet took to reach that network segment.

Common Routing Problems

If you suspect that your RRAS server isn’t functioning properly, start by making sure the RRAS server is running. You might be surprised how many times the cause of the problem turns out in fact the that RRAS is not turned on.

Most TCP/IP administrators spend much of their time troubleshooting the hardware. Connectors go bad, NICs die, and cables break or are cut. You need to troubleshoot and repair these elements before you start looking at the software. Consider these potential trouble spots first:

• Check for basic communication between systems first. Broken cables, loose connections, and so on can cause what might look like much more complex problems.

• Make sure that your systems are in compliance with the standards you’ve chosen. This means you need to verify all devices on your Ethernet are broadcasting Ethernet and not something else. Make sure you have the correct types of cables. An example of this is the common mistake beginners sometimes make using RG59A/U cable instead of RG58A/U. The former cable type is used in broadcasting specifically with video; the latter is used with IEEE 802.3 10Base2 networks.

• Carefully isolate your problem to a single LAN, MAN, or WAN segment by going through each individually. Keep in mind it is extremely rare for two segments to go down at the same time.

Interface Configuration Problems

Make sure that the RRAS server is configured to perform as an IP router. Open the RRAS Microsoft Management Console (MMC) and verify all your settings. Make sure that you have enabled RRAS on the Windows Server 2003 machine you are expecting to perform as a router. It could be that you have the wrong server configured. Also, keep in mind that the system must first make the physical connection to the network. After that, it must make the logical connections.

The router also might not be receiving routed data from other routers. Take a look at the routing table to see that the router is receiving routes from the other routers. If there is anything there other than Local in the Protocol column, the router is receiving routes via the routing protocols. If not, double-click the rest of the settings in this section and pay particular attention to the appropriate protocol.

RRAS Configuration Problems

Routing for the correct LAN protocol may not be enabled. If you’re using IP routing, make sure that IP routing is enabled on the IP tab of the server’s property sheet. Also, make sure that you have IP routing protocols attached to each of the interfaces where they are needed.

The wrong protocol could be installed, or the right protocol could have been installed on the wrong interface. The correct protocol must be installed on the appropriate interface for this to work correctly.

Routing Protocol Problems

One of the most common problems you’ll face with RIP for IP is incorrect routing table entries. If you’re seeing wrong or inconsistent routes in the routing tables, or if routes are totally missing, you should look at the following possibilities:

• The wrong version of RIP could be in use.

• Silent RIP hosts might not be receiving updates.

• The subnetting scheme on your network could be incompatible with your routing infrastructure.

• A router might be using the wrong password.

• Routing filters might be too restrictive.

• Packet filters might be too restrictive.

• Neighbors might be incorrectly configured.

• Default routes might not be being propagated.

If your router is using OSPF, make sure that the Enable OSPF on this interface check box is selected. This option is in the interface’s OSPF Properties dialog box.

Also make sure that your router is receiving routing information from the other routers on the network. Do this by opening the routing table and looking at the Protocol column. One of the following might be the problem with OSPF:

• OSPF might not be enabled on the desired interface.

• The neighboring router might be unreachable.

• The OSPF settings may not match on each of the neighboring routers.

• The stub area configuration or area ID on neighboring routers may not match.

• Interfaces may not be configured with OSPF neighbor IP addresses.

• There may not be a designated router (DR) for the network.

• Packet filtering may be too restrictive.

• Summarized routes may be configured improperly.

• ASBR source or route filtering may be too restrictive.

• Virtual links may be incorrectly configured.

If a routing table entry is marked as being either OSPF or RIP, then information from some of the other routers on your network is getting through. If you do not see any OSPF or RIP entries in the table, you have a problem.

TCP/IP Configuration Problems

EXAM 70-293 OBJECTIVE 2.5.3

Verifying that the router’s TCP/IP configuration is correct first may save you a lot of time. You must use the correct IP address and subnet mask.

Routing Table Configuration Problems

You’ll need to have a static default route defined and enabled so that your router will forward any packets when there is no specific route designated for them. If the default route is incorrect or missing, you will have problems. If you’re using default routing, the default route must be learned through the routing protocols or statically configured on the router over the correct interface.

Test Day Tip

You will need to know extremely basic problems and their solutions for the exam.