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CLARIFICATION - PING LOOPBACK OSI LAYERS

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maczen

maczen

Instructor
Apr 12, 2008
1,016
US
Quick question..

When I ping the loopback address of:
A) Cisco Device (icmp traffic actually sent on wire)
B) PC (No traffic on wire)
What layers of the OSI model are being tested?

Here is the reason that I ask...

I have heard 1-3, 3-4, 1-7 etc.

I believe that the same layers are being tested regardless of whether or not the traffic crosses the wire but looking for someone to enlighten.. LoL Thanks

B Haines
CCNA R&S, ETA FOI
 
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PING is 1 - 3. Telnet hits the application.

[the other] Bill
 
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Just because it tests the "TCP/IP stack" of a Windows station, this is sort of a misnomer. Bill is correct---layers 1-3 are used when pinging the loopback---physically, the ICMP packet travels on internal pins on the NIC/ethernet controller to simply test L1 through 3 functionality. If it works, then it is said that the "TCP/IP stack" is intact, but in this case it can be separated into just the "IP stack". The "TCP/IP stack" is a term made up by a bunch of Micrps#$t techs who know nothing about networking, and therefore it has been assumed that since TCP is mentioned in the term, then a loopback ping must include layer 4. Survey says...BAAAAAAAAAAAAAAAAAA!!!!!
Granted, there are some ICMP error messages that utilize L4 protocols, but ICMP resides at layer 3.
Why wouldn't they call it the TCP/IP/UDP/IPX/SPX/DECNET/MOP/ICMP stack?lol

Burt
 
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Billy,

I always heard and was told when I asked that "Ping" is used because it is very easy for a tech to say "ping 127.0.0.1, then ping the default gateway 10.10.10.1, then ping the DNS server 10.10.145.56" etc.

Whereas telnet is a more complete test, if I can telnet to another machine - PC, router, switch etc. I am going up and down all 7 layers of the OSI or all 4 layers of the TCP/IP model :) but as a tech I might have to try to teach the person on the other end a little about networking if I use the term telnet :-(

E.A. Broda
CCNA, CCDA, CCAI, Network +
 
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Gene,

I think the question from Billy was based on the thread he referenced. There is some confusion in that thread, enough to maybe allow a little doubt to enter into the mind of even such an expert as Billy.

(Nice explanation, Gene. Only being a raw rookie, I was reluctant to get into a more detailed discussion with the gurus.)


[the other] Bill
 
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A good way to learn is Ethereal/Wireshark. Fire it up, and ping the localhost.

Burt
 
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Gotcha.. Thanks for the information guys! Now as per that link above an individual said that a loopback ping on a Cisco device hits all seven layers because it foes out over the wire.. This is also incorrect yes? Still 1-3???

B Haines
CCNA R&S, ETA FOI
 
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Yes---1 through 3. Billy---fire up Wireshark and ping your router. You will only see L2 and 3.
Another interesting thing is the 32 bytes that it sends---look at exactly what it is...post back with the answer...

Burt
 
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Ping can be really interesting to see what the data it sends is. I've never captured from a cisco device, I know windows sends the alphabet, and I remember once I had a ping app that would send "the quick brown fox, ..."
 
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ok.. thought I was going nuts but you can not capture out of the box on Windows..


Need the Microsoft loopback adapter

I need to research this a little.. support.MS has instructions for XP, Server 2003 etc. but not Vista (although I have found some third party sites but all from 2006 (pre serv pack one) .. Do not want to be troubleshooting the Windows partition today! Will run the wireshark scan on Linux later on or tomorrow!

B Haines
CCNA R&S, ETA FOI
 
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data packet shows abcdefghijklmnopqrstuvwabcdefghi is what the 32bytes shows up when I ping the local router, but I can not get it show anything when I attempt to ping localhost or 127.0.0.1

------------------------------------
Dallas, Texas
Telecommunications Tech
CCVP, CCNA, Net+

CCNP in the works
 
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Billy,

Funny thing is that on my first CCNA exam (I think in 2000) I had the question like What does pinging the loopback 127.0.0.1 tell you :) one of your posts at your link above could have been one of the answers word for word!!!

E.A. Broda
CCNA, CCDA, CCAI, Network +
 
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Dallas---it won't work, since it's internal. I made a mistake when I told Billy to ping the localhost and capture the packets. The packets must actually go out and back in, which is why an ethernet loopback would work (xmit wired to rcv). They are easy to make---just Google the pinout.
Gene---I had the same question, and I think I actually had that on CompTIA Network+

Burt
 
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Ah ok... ya, i have a few of those loopbacks and crossover adapters laying around, they really come in hand when you are in the field =) found one at a fry's here that has a male and female on it.

------------------------------------
Dallas, Texas
Telecommunications Tech
CCVP, CCNA, Net+

CCNP in the works
 
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Thanks again guys!

Burt,
I honestly can not ping the loopback... have installed MS loop adptr twice.. LoL No luck! Will just do it on Linux later this week.. Study time now! LoL

B Haines
CCNA R&S, ETA FOI
 
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uh huh huh huh huh...you thaid microsift...huh huh huh huh...

You can't ping it WITH the loopback plug, without it, or neither?

AT any rate, ping is ping---ping from the router to your computer with Wireshark running.

Burt
 
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OK, I'll jump in on this.

Ping loopback tests the TCP/IP protocol stack...all layers. Hence, it tests all layers of OSI since the TCP/IP model roughly maps to OSI. Normally, for testing remote devices, we consider ping as testing only up to Layer 3. But the loopback is different. In the context of Cisco and CCNA certification, ping loopback tests L1-7.

If you want to open another can of worms, where does ARP fit in relation to OSI? Cisco puts it at L3. Some vendors place it at L2.

HTH
 
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Clue,

I'm interested also. Would you mind posting a link?

TIA,

[the other] Bill
 
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This is from MS:


"Use the PING command to verify that TCP/IP is working properly. To do so, ping the loopback address (127.0.0.1) by typing the following command at a command prompt:
ping 127.0.0.1
You should receive a response similar to the following:

Pinging 127.0.0.1 with 32 bytes of data:

Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128


If you receive an error message at this point, TCP/IP is not properly installed. To remove and reinstall TCP/IP, follow these steps: "

Since TCP/IP layers map roughly to OSI, then for Cisco, you're testing all OSI layers when you ping loopback.
 
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