Troubleshoot Cisco routers and switches using the debug commands - Part 2

Let’s take a look at a simple example. We are going to view RIP (Routing Information Protocol) in Debug mode.

Router# debug ip RIP

RIP protocol debugging is on

To verify what debugging is enabled, use this command:

Router# show debug

  RIP protocol debugging is on

The output from whatever type of debug is enabled will be sent to wherever the Cisco IOS logging system tells that output to go. Either you will receive the output on your screen, it will go to the buffered log on the router, or it will go to a syslog server across the network (or all of these).

To see what level the various outputs are set to and where the output will go, type:

Router# show logging

Syslog logging: enabled (1 messages dropped, 3 messages rate-limited,

                0 flushes, 0 overruns, xml disabled, filtering disabled)

    Console logging: level debugging, 8 messages logged, xml disabled,

                     filtering disabled

    Monitor logging: level debugging, 0 messages logged, xml disabled,

                     filtering disabled

    Buffer logging: level warnings, 2 messages logged, xml disabled,

                    filtering disabled

    Logging Exception size (4096 bytes)

    Count and timestamp logging messages: disabled

    Trap logging: level informational, 12 message lines logged

Log Buffer (51200 bytes):

*Jun  9 20:56:49.195: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

*Jun  9 20:56:49.231: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up

Router#

The console should display RIP updates that are sent and received through the RIP protocol. Here is an example of what you might see for RIP debugging:

*Jun  9 21:13:56.471: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (1.1.1.1)

*Jun  9 21:13:56.471: RIP: build update entries - suppressing null update

*Jun  9 21:14:22.519: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (1.1.1.1)

*Jun  9 21:14:22.519: RIP: build update entries - suppressing null update

Remember that you should use Debug only for a short time to get a snippet of information, and then turn Debug off as it can be a serious performance hit on your router.

There are several commands for turning off Debug.

Router# no debug

If you type debug ?, you will see that there are over 200+ Debug commands, and each of those has many options. Debugging RIP is just a very simple example.

Troubleshoot Cisco routers and switches using the debug commands - Part 1

Often, you cannot solve router and switch issues with “show” commands alone. At some point, you will have to use Debug commands to find out what is really going on behind the scenes. In this article, you will learn about Cisco IOS Debug commands and how they can help you.

What makes Cisco IOS Debug commands so useful?

Cisco IOS Show commands can tell you many things about what is going on with your router or switch, but they can’t tell you everything. For example, Show commands cannot tell you when routes drop in or out of the routing table, why an ISDN line failed to connect, whether a packet really went out the router, or what ICMP error code was received. On the other hand, Cisco IOS Debug commands can tell you all these things, and more.

Besides providing more detailed information than what Show commands can provide, Debug commands have the benefit of providing information in “real time” (or dynamically). This is contrary to Show commands that just take a snapshot in time and display the results on your console (somewhat static results). This real-time difference can be very helpful in diagnosing problems.

How to learn CCNA efficiently?

Following is some comments on how to learn CCNA efficient. I think it is helpful.

1. I bought both Lammle's book, plus the Cisco PressICND1, and ICND2 books by Wendell Odom

I like Odom's books and as there are two (smaller) ones, I can carry one and read on the train/plane. I prefer the style to Lammle's book, but I'm sure mileage varies.

You should also consider doing a CCNA course or bootcamp. I recently attende one. In the interest of fair disclosure, I work for Global Knowledge!

2. I think the best advice I can give you regarding the 640-802 is to subnet, subnet and then subnet some more. Only when you have a true grasp on the complexities of subnetting and general IP addressing schemes can you really begin to appreciate other topics.

The CISCO press books are a little dry but they are packed with the information required. I would probably then plump for a ExamCram volume as a reference point in the last few weeks before the exam. These books will not teach you the topics in any great depth but they can act as good benchmarks.

3. Lammlee's book is a good resource. I also used the Transender practice exams and purchased a couple of routers and switches off ebay to practice on. I was able to pass the ICND1 on the first attempt using only these resources. (I decided to go the two-exam route)

The ICND2 exam is a little tougher IMO. Make sure you understand spanning tree, OSPF, NAT/PAT.

4. Odom's (Cisco Press) ICND1 and ICND2 and both are excellent.

I used Odom's ICND1 to prepare for the 1st exam. I followed his study plan recommendations and did very well on the exam.

I'm using Odom's ICND2 now to prepare for next exam and my studies appear to be going well. Below is a summary of what I'm doing.

1. I'm using both the ICND1 and ICND2 texts to prepare as if I'm studying for the composite (640-802) exam. I've found this helps me better understand topics covered in ICND2 text.
2. After I read a topic in ICND2, I go back through the material and practice the commands and configurations on my home lab. I go over and over the 'show' commands used.
3. I spend 3-4 hours a week getting more hands-on to drill in what I've covered to date.

I tend to recall commands and show output much better from hands-on than from the text. I may be wrong but it seems that way to me.

I purchased Lammle's text for the composite (640-802) exam thinking I would read it in conjunction with the Odom texts. Again, maybe it's just me, but that didn't work well for me. In fact, I pulled Lammle's text off the shelf tonight to read his section trunking, vtp, etc. I had already covered the material in Odom's text and I thought Lammle's would help me catch anything I missed. Didn't work. In fact, I stopped and put it back on the shelf because I didn't want it to confuse me. (It also appears to me that Lammle doesn't devote as much time as I need to some topics.)

5. There are a number of resources, Mirza - to some degree it depends on how you learn best.

I've posted above two sets of books (Lamie and Odom) that for me are very good. I guess these are a great starting point.

In addition to this, you might consider taking a training course. I'm biased because I work there, but take a look at Global Knowledge's classes (www.globalknowledge.com, or www.globalknowledge.net). Depending on how much you can do youself, consider the CCNA bootcamp vs the ICND1/ICND2 classes.

Getting some real hardware or a good simulator is also important. Boson's product looks pretty good, and there'a an open souce product (but that needs actual IOS images that may not be readily available to you). But picking up a couple of switches and a router should not be too expensive (ebay to the rescue).