Static routig HELP ASAP PLEASE

Answered Question
May 26th, 2007

I have four routers connected like a hub on spoke topology. The central router is Denver and is connected to LA, Houston and Miami by serial links. All have their connected interfaces correctly configured. Static routes are configured : Denver has a static route to each ethernet subnet on Miami, LA and Houston. These three routers have a default route to Denver (See configuration on the attach file).

The question is :

Can a host in Miami ping the serial interface S0 in LA. (Assume that the serial S0 of LA is connected to Denver and denver is connected to Miami)

The book's author answer NO.

But i think yes, because the default route for Miami is to sent its packets to Denver, and as it is directly connected to LA, it can forward the packet toward the S0 interface of LA. Then LA can reply to denver which is directly connected.

Finally, as static routing is configured on this last router it can forward the packet to the destined host via Miami.

Help me fix that.

I draw a map and attach. Should you need to see the topology.

Attachment: 
I have this problem too.
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Correct Answer by CSCO10892433 about 9 years 6 months ago

Hi, ferell

You are quite right on your analysis about the author's errors. The next-hop address could be a typo, the answer should be YES, and the reason that the author gives you is explaining why the answer is YES rather than NO.

You are doing well on analyzing questions like this one, and such an abliltiy is very important and helpful to answer question on real exam. But, please remember, there is very few typos or errors you can find in the real exam. If there is a wrong ip address, that is a wrong ip address. Base on that information to analyze the question and choose the best answer accordingly.

Please also notice the time you spend for each question. Don't spend too much time on a tricky question like this. Try to complete all the questions before the time is over. You might loose one or two tricky questions, but you still have a good chance to pass the exam.

Good Lucks

SSLIN

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CSCO10892433 Mon, 05/28/2007 - 21:40

Hi, farell

Your analysis is excellent and I agree with you that , with all the clues this question has given, there is no reason to say that Miami cannot ping LA's S0 interface.

If this is not just a qustion, but a real life senario, I would like to ask: Is there any Layer2 issue? Does any IP address has an adequete Layer2 mapping? Also notice that the mask that hub and spoke use are different ( hub uses /30, spokes use /24) , this might get problems with some dynaic routing protocols (like OSPF), but in this senarion, it uses static routing, so it won't get any problem.

The book's answer is no. Does the author give any reason?

HTH

SSLIN

PS: In my original post, I give an explaination about Layer3-to-Layer2 maping on encapaulation type of frame-relay. It is tedious and, in my opinion, beyond the scope of CCNA. So, I deleted it finally. If you are interested, let me know.

farellfolly Tue, 05/29/2007 - 02:20

I've just seen the author explaination. He's wrong about himself beacause the question was about to choose between A (yes) B (no).

He has chosen B but he argue in that way :

The Miami and LA routers have both default routes, and the Denver router knows about all subnets in the network (some connected, some through static routes).

Don't you think he's talking the same language as us even though he's written down B(no)? (If yes, i am sorry, i wish i had not posted this question but i was anxious about the answer B before i finally find his explaination)

However, i am interested about reading the frame-relay encapsulation type. It could help me

CSCO10892433 Tue, 05/29/2007 - 02:52

Hi, farell

Another issue I did not notice before is that the source address of the ping is from Miami's ethernet subnet 10.1.2.0/24. When LA receives this packet, it will try to reply that packet. The destination addresss will be 10.1.2.0/24 which is in a different subnet, but the same netwok number 10.0.0.0, from it's ethernet subnet 10.1.4.0/24 . This might be a problem if you are running classful routing on the LA router. Classful routing's algorithm is: match the major network first, if there is a match, match the subet second, if there is no match, drop the packet.

Could this be a reason why the author answer no? Does this question provide any information about "ip classless" command ?

HTH

SSLIN

farellfolly Tue, 05/29/2007 - 03:45

The author did not mention nothing about "ip classless". This option, isn't it enabled by default?

In this sentence from you :"Classful routing's algorithm is: match the major network first, if there is a match, match the subet second, if there is no match, drop the packet"

Don't you want to say : if there's not a match, match the subnet second?

PS: i am french-speaking, so sometimes i could miss something in the explainations. I understand classfull issues in routing and the purpose of the ip classless command but you can do it again for me..

However i dont think it will influe that situation which is dealing with static address and DEFAULT (0.0.0.0 = any) routes.

Hope to hear from you soon

thx, farell

PS: i will design the whole network in Boson and try to ping and tell you later. If you work with routers at work, can you please try it for me? It is always better than simulators as far as they can bug for another reasons.

CSCO10892433 Tue, 05/29/2007 - 04:02

Hi, farell

Yes, I will try your network on my home lab tonight. I will give you feedback tomorrow. Please wait.

Regards

SSLIN

eugene.fourie Tue, 05/29/2007 - 04:06

The setup that you created doesn't work, I have tested this in a lab, i have changed some of your settings which include /24 networks and the ip route statement on Denver for 10.2.1.0 as this should be to interface 172.16.0.2.

The packets are destined to the correct ip when you run a debug on the IP packets, but for some reason it does like the fact that there are sub-interfaces.

If someone could assist with an explanation.

farellfolly Tue, 05/29/2007 - 07:38

Hi eugene, you're right about the statement on Denver to reach 10.1.2.0 (not 10.2.1.0) through 172.16.0.2 instead 172.16.0.1 which the ip address of the S0.1 interface of Denver.

But the thing i wrote down was directly from the author's book. But i do agree with what you say.

Anyway that does not influe on the PING!

PS: If i can have answer before my exam, that could be great. I will be passing it on Thursday 31st at 14h00 (Belgium : greenwich +2)

CSCO10892433 Tue, 05/29/2007 - 18:57

Hi, parell

I also noticed the wrong next-hop ip address last night and I think it is so far the best reason to explain why the answer is No. Thanks to eugene's finding :-)

Anyway, I did test this question base on the correct next-hop ip address (my router wouldn't accept it's own ip address as the next-hop). The answer is Yes and I list all the command necessary, including Layer2 and Layer3, to make the ping successful on the attached file.

As for the classful routing algorithm, here is a brief explaination:

1.match the Major network first. If there is a match, goto step 2. otherwise, goto step 3

2.match the subnet of the matched major network. If there is a match, forward the packet accordingly, otherwise drop the packet.Goto step 4.

3.If there is a default route, forward the packet accordingly, otherwise, drop the packet.

4.End

As you can see, the default route will only be used when there is no match in major network.

I will explain frame-relay map later.

Finally, wish you success to your CCNA exam.

BR

SSLIN

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farellfolly Wed, 05/30/2007 - 05:09

Should i understand that if there were no wrong next hop ip address on Denver, the ping should have worked?

For a cisco CCNA i am afraid about thoses questions!! Because without a simulation, you could miss something.

You would have noticed that the next hop on S0.1 was wrong but the ping is not toward this interface at all, and the interfaces which are concerned are all well configured.

In CCNA exam you would thus want to say yes because of the good configuration of the right interfaces but NO because a router does not accept its address as next hop (But unfortunately the question does not talk anything about it and may be it's JUST AN ERROR OF WRITING WHEN USING A KEYBOARD ==> see author explaination)

Anyway, despite the right answer of the author (NO)according to your simulation, i think his explaination is not helpfull : miami and LA routers have both default routes, and Denver knows abot all subnets in the network : some connected, some through static routes

WHEN THE AUTHOR SAYS : "knows about all subnets in the network", I think he is not considering the mistake on DENVER (wrong next hop IP)

Any suggestion? answer ASAP, please

THx all, i hope i will success

Correct Answer
CSCO10892433 Wed, 05/30/2007 - 17:55

Hi, ferell

You are quite right on your analysis about the author's errors. The next-hop address could be a typo, the answer should be YES, and the reason that the author gives you is explaining why the answer is YES rather than NO.

You are doing well on analyzing questions like this one, and such an abliltiy is very important and helpful to answer question on real exam. But, please remember, there is very few typos or errors you can find in the real exam. If there is a wrong ip address, that is a wrong ip address. Base on that information to analyze the question and choose the best answer accordingly.

Please also notice the time you spend for each question. Don't spend too much time on a tricky question like this. Try to complete all the questions before the time is over. You might loose one or two tricky questions, but you still have a good chance to pass the exam.

Good Lucks

SSLIN

farellfolly Wed, 05/30/2007 - 23:54

Thx to SSLIN,

Thx everybody

I'll be passing in four hours...

hmm

Tell you later

farellfolly Thu, 05/31/2007 - 07:26

I've passed my CCNA. My score is 949

Per section :

Planning an designing : 100%

Implementation and operation 91%

Troubleshooting: 88 %

Technology 92 %

For troublesooting, i think i've been working with routers just when i start to study for CCNA (Last April 23rd) and i did not use to manage routers in real-time in real life (Because i've just finished my five university years for telecom engineer grade)

Hope i'll put hands on working with routers and get more skills and abilities

Thx everybody, i'll dedicate a topic for ya all

Farell

guillaume.labarthe Sun, 06/17/2007 - 04:30

Hello,

In my opinion, the reason for "NO" is really simple :

Miami-S0 is a /24 mask (172.16.0.6/24), so, when pinging 172.16.0.X, the router looks for a matching arp entry, because this network is directly connected. It does not find it so cannot ping 172.16.0.10.

Does anyone agree with me ?

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