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Home > Archive > CCNA > April 2004 > OSI model question
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OSI model question
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| repentantvoter 2004-04-28, 9:11 pm |
| Good evening ladies and gentlemen! I have found this question is a practice test, and I have failed to find the correct answer! 
You used the PING utility to test IP connectivity between two routers. If the PING is successful, which OSI layers have you tested?
a. Layer 1, Layer 2, Layer 3
b. Network and Data Link layer
c. Layer 3 only
d. Network and Transport layer
e. Layer 1 thru Layer 7
My answer was e; at the end of the test I was informed the correct answer is a. My line of thought was that the "PING utility" is the program commonly encountered on most operating systems. I read somewhere that this type of networking programs do span all the layers of the OSI model indeed, since they present an interface to the user (application layer), may have to do encoding conversions (presentation layer), etc. However it is apparent that the author of this question was considering only the "echo request" and "echo reply" messages belonging to ICMP.
My question is: do you think I am reading too much into the question, thus missing the essential? Or is the question wrong indeed, and my answer correct?  | |
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| Sexy Lexy 2004-04-29, 1:00 pm |
| It depends, LAN uses Layers 7 to 1 but WAN uses Layers 3 to 1.
It depends upon the target host but I would have to agree with Darth and say "A"
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| Goldorak 2004-04-29, 5:47 pm |
| Well, new to it all, but if i may:
My choice would have been answer A
Reason:
Ping tests Physical Layer - ie: Damage cable: no repsonse..
Ping tests Layer 2:
Ping tests Layer 3:
What about layer 4? Sureley reliable (TCP) and unreliable (UDP) delivery is part of the test!
What about layer 5: Surely a session has to be created between the testing node and the tested node?
...
Actually, i'm quite interested to know what you guys think about this...
Thank you | |
| edmonds_robert 2004-04-29, 9:08 pm |
| quote:
Originally posted by Goldorak
What about layer 4? Sureley reliable (TCP) and unreliable (UDP) delivery is part of the test!
What about layer 5: Surely a session has to be created between the testing node and the tested node?
Thank you
Layer 4 isn't tested because ping doesn't use TCP or UDP, it uses ICMP, which resides at layer 3.
Layer 5 isn't tested because a session is not opened. A packet is sent, and a reply is sent, but that does not necessarily consitute a session. | |
| perfectionist 2004-04-30, 2:16 am |
| I agree with the way edmonds_robert has explained..
"Layer 4 isn't tested because ping doesn't use TCP or UDP, it uses ICMP, which resides at layer 3."
"Layer 5 isn't tested because a session is not opened. A packet is sent, and a reply is sent, but that does not necessarily consitute a session."
If all ports of a host are closed or no session is established between the sending host and the receiving host, you can still ping the host and get a positive reply as long as the other 3 lower layers are working. | |
| Goldorak 2004-04-30, 2:20 am |
| Nicely put by perfectionist. It does make sense.
Thank you | |
| repentantvoter 2004-04-30, 5:12 pm |
| A very instructive discussion indeed!! For me the conclusion is that I need to capture the essence of the question. I shall try harder to do just that! Thank you all very much! |
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