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CCNA Practice Exam :

EIGRP

Chris Bryant, CCIE #12933

 

Here's a practice exam and tutorial on EIGRP for you CCNA candidates - and it wouldn't hurt you CCNP candidates to take it!

CCNA candidates should also take our EIGRP Fundamentals practice exam.

In this exam, we're going to start with some EIGRP basics, and those will be followed by a scenario-based question. That scenario-based question will in turn lead to a tutorial that CCNA and CCNP candidates alike will benefit from.

Let's get started!

Question 1:

You've just finished an EIGRP configuration, and you're not seeing quite the number of subnets you expected in the EIGRP routing table. Odds are you left out a command in your config. Which command would that be?

 

Question 2:

Of VLSM, IPX, IP, and AppleTalk, which is/are supported by EIGRP?

 

 

Question 3:

What two metrics are used by EIGRP's routing algorithm to calculate routes?

 

Question 4:

What is EIGRP's routing algorithm, anyway?

 

Scenario Question:

According to your EIGRP topology table, there are four loop-free paths to a single network. The metrics are:

Path 1: 1000

Path 2: 2345

Path 3: 6234

Path 4: 7100

Using that information, answer the following questions:

 

Question 5:

How many successors are there?

 

Question 6:

How many feasible successors are there?

 

Question 7:

Do paths 2, 3, and 4 appear in the EIGRP routing table?

 

Question 8:


If not, how can we make paths 2 and 3 appear in the EIGRP table without bringing path 4 in?

 

Answers right after this brief and important message!

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I told you it was brief. And important!

 

Answers:

1. If you're not seeing the subnets you think you should be seeing, it's likely you need the no auto-summary command added to the EIGRP config.

 

2. EIGRP supports all four - VLSM, IP, IPX, and AppleTalk.

 

3. By default, EIGRP's routing algorithm uses bandwidth and delay to determine route metrics....

 

4. ... and that routing algorithm is DUAL, the Doubly Unbelievable Actual Louse.

Okay, it's really the Diffusing Update Algorithm. But I like my version better.

 

5. The path with the best metric - in this case, Path 1 - will be the single Successor.

 

6. I mentioned that all four paths were loop-free and valid; that means that all non-Successor paths will be Feasible Successors.

 

7. By default, only the Successor routes will appear in the routing table. Both the Successor and Feasible Successor routes will appear in the EIGRP topology table, though.

 

8. Notice I said "by default". We can use the variance command to bring Feasible Successors into the routing table.

 

In this case, we want to bring Paths 2 and 3 into the routing table without doing so for Path 4.

 

We'll use these metrics to do so...

Path 1: 1000

Path 2: 2345

Path 3: 6234

Path 4: 7100

... and we'll use the variance command.

The variance command is one of those commands that sounds complicated when you hear or read the theory, and then when you do it, you say "Hey, that was easy!"

That's exactly what you'll be saying in just a minute.

The variance command is just a multiplier. When you configure it, the router multiplies the metric of the Successor and then puts all Feasible Successors with a metric smaller than the result of the multiplication into the routing table.

I told you it sounded complicated. Any paragraph with three variations of the word "multiply" can't be good!

Let's practice with this example and you'll see it's quite simple.

We can take a look at that metric of 1000 and quickly figure that we need a variance of 6 to bring paths 2 and 3 into the routing table.

The result of entering variance 6 into the EIGRP config:

DUAL looks at metric of our Successor, which is 1000.

Result of 6 x 1000: 6000

All Feasible Successors with metric of 6000 or less will now be put into the routing table. That's Path 2... and no others.

Whoops! Looks like we need a variance of 7 instead. Let's do that...

DUAL looks at metric of our Successor, which is 1000

Result of 7 x 1000: 7000

Now both Paths 2 and 3 will be put into the routing table, while Path 4 will not.

The great thing about the variance command is that you don't need a laptop or practice exam to practice it - just a piece of paper and a pencil.

Or a pen, if you're as hardheaded / confident as I am!

 

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