Correcting Problems Occurring at the Transport and Application Layers

This section details a brief yet beneficial procedure to be followed when correcting problems occurring at the transport and application layers (or in general). Because the order of execution is important, the steps are presented as a numbered list:

1. Be sure that you have a saved configuration for the device whose configuration you are about to change. During the course of troubleshooting, you should always be able to revert back to a known initial state. Casually speaking, you want to make sure that you do not make matters worse than they were originally.

2. Start making changes that you have decided are necessary to correct the problem. However, make one atomic change at a time.

3. Evaluate your change(s). If the results are not good or as expected, undo the changes.

4. Verify that your changes did not introduce new problems. The goal is to return the network to its baseline, so no new or old symptoms must remain. If you cannot rectify the situation, undo your changes.

5. Continue making changes until the problem is solved.

6. You might need to seek assistance from outside resources, such as a co-worker, consultant, or Cisco Technical Assistance Center. In certain troubleshooting cases, a core dump might be necessary; therefore, take the necessary steps to familiarize yourself with the procedure for performing a core dump. Specialized Cisco Systems personnel analyze core dumps.

Document your solution and all your changes.

Foundation Summary

The "Foundation Summary" section of each chapter lists the most important facts from that chapter. Although this section does not list every fact from the chapter that will be on your CCNP exam, a well-prepared CCNP candidate should at a minimum know all the details in each "Foundation Summary" before taking the exam.

Table 12-3 IP Access List Commands Useful for Correcting TCP and UDP Problems

Command

Description

access-list {access-list-number} {deny | permit} {ip | udp | tcp | . } source-address source-wildcard destination-address destination-wildcard [operator operand] [log]

Syntax for defining an extended access list. Allows you to specify more precise filtering conditions. You can check source and destination IP address (with wild-mask). Allows you to specify protocol and port number.

ip access-list {standard | extended}

{access-list-name}

Syntax for defining a standard or extended named access list.

ip access-group {access-list-number | access-list-name} [in | out]

Entered in interface configuration mode, this command applies an access list to the interface. Inbound traffic is affected if the trailing keyword in is used. If no keyword is entered or if the out keyword is used, the access list affects outbound traffic.

Table 12-4 Commands Used to Correct Application Layer Problems

Command

Description

snmp-server enable {informs | traps}

Enables SNMP informs or traps.

snmp-server community name [rw | ro] [access-list-number]

Enables SNMP and sets community string. The standard or expanded access list that is referenced optionally specifies the address range of the IP hosts that are permitted to have read-write (rw) or read-only (ro) communication with this device.

snmp-server host {name | IP-address}

Specifies the host name or IP address of the SNMP notification host that is receiving traps.

ntp server {ip-address}

Specifies the IP address of another device that will act in the capacity of an NTP server.

ntp peer {ip-address}

Specifies the IP address of another device that will have a peering relation with the local device.

Table 12-4 Commands Used to Correct Application Layer Problems (Continued)

Command

Description

ntp source {interface}

Specifies which interface's IP address should be used during NTP communications with other devices.

Service timestamps log datetime localtime

Configures the router to timestamp log messages with the local date and time.

service timestamps debug datetime localtime

Configures the router to timestamp debug messages with the local date and time.

ip helper-address {address}

This is an interface configuration command, which means different interfaces can have different settings. The UDP broadcasts (with certain destination ports, such as 68, 67 for BootP) are converted to unicast and sent to the IP address specified.

[no] service dhcp

Enables/disables DHCP server and relay-agent functionality on the local router. When DHCP service is enabled, the related configuration parameters are then set with the ip dhcp global command.

Table 12-5 Support Resources for Correcting Transport and Application Layer Problems

Resource Name

Universal Resource Locator (URL)

Cisco Systems Technical Assistance Center

www.cisco.com/tac/

CCO

www.cisco.com

Cisco Systems Technologies Reference

www.cisco.com/univercd/home/home.htm

IETF

www.ietf.org

IRU

www. itu.int/home

FRF

www.frforum.com

ATM Forum

www.atmforum.com

You should have the following information gathered before contacting Cisco Technical Assistance Center:

■ Complete network diagram, or at least the affected area. The IP address/mask of the IP devices should be shown if possible.

■ All the information and any facts gathered.

■ The output of the show tech-support command if the number of affected routers is fewer than four.

■ Dial-in or Telnet access to the devices under investigation.

Table 12-6 Guidelines for Correcting Problems Occurring at the Transport and Application Layers

Step

Description

1

For the device(s) whose configuration you intend to change, ensure that there is a saved valid configuration.

2

Make the intended changes. Make one change at a time.

3

Evaluate and document the results of the changes made.

4

Verify that the changes made did not introduce new problems/symptoms.

5

Continue making changes until problems are fixed.

6

Seek assistance from outside sources, such as other colleagues, consultants, and Cisco's Technical Assistance Center, if necessary.

7

Document the solution.

Q&A

As mentioned in the introduction, you have two choices for review questions. The questions that follow give you a bigger challenge than the exam does because they use an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. You can find the answers to these questions in Appendix A.

For more practice with exam-like question formats, including questions that use a router simulator and multiple choice format, use the exam engine on the CD.

1. Which Cisco IOS command applies an access list to an interface?

2. Which Cisco IOS command enables SNMP and sets community string?

3. List the commands that configure the NTP server and the NTP peer.

4. Which Cisco IOS command configures the router to timestamp log or debug messages with the local date and time?

5. Which Cisco IOS command enables (or disables) DHCP server functionality on the local router?

6. Which Cisco IOS interface configuration command converts the UDP broadcasts (with certain destination ports, such as 68, 67 for BootP) to unicast and sends them to the IP address specified?

7. What are the URLs for the Web pages of the IETF, ITU, FRF, and ATM Forum?

8. What are the URLs for Cisco Systems Technologies Reference, Cisco's Technical Assistance Center, and CCO?

9. What information must you gather and have ready before contacting Cisco Technical Assistance Center?

10. List the seven steps to correct problems occurring at the transport and application layers.

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