arp and rarp questions....

Chris Knight

The techexams note that both these protocols exist at the data link layer 2 level.
However, I am reading the exam cram 2 test book, and they state that both protocols exist at the network layer 3 level.
Can anyone clear this up for me? Im all confused

Danman32

I'd say it's in-between layer 2 & 3. Arp is used to discover and map a MAC address (layer 2) to a known IP address (layer 3). Rarp is the opposite, to map a known MAC to an IP.

Looking at an ARP frame, I'd have to say it is more layer 3 than 2, as it builds on top of the Ethernet frame. But instead of using an ethernet datatype of IP (0x0800), it uses its own datatype ARP (0x0806).

Chris Knight

Right I would think as well that arp and rarp reside in layer 3. Seeing as how were dealing with IP addressing and that resides at layer3.

So for the record if asked at the testing facility arp and rarp reside at layer 3

Arp=resolves IP addressing layer3 to Mac addresses which are layer 2.

Rarp=resolves mac addressing to IP addressing

I hope they dont ask me this question.

I have been reading the technotes, and all seem to be on par, however the books im reading specify otherwise...

Danman32

Yup, I hate questions that can be answered more than one way depending on how you look at it.

What is this picture?

1. A vase
2. Two people kissing
3. Paint splatter
4. Doodleing.

I say #2, but if you squint, you might see #1.

agustinchernitsky

Hello,

ARP is a level 3 (network) protocol. It is used to resolve IPs to MAC address. If the MAC address is not found in the ARP cache, then, the protocol broadcasts the entire network to find the corresponding MAC address (using layer 2).

It's something like "Who has 192.168.0.10?"... and the owner of the IP will respond "Me, my address is xxx:Xxxxx"

RARP is the other way... used mainly for bootp or something like that!

Hope it helps!

Chris Knight

agustinchernitsky wrote:

Hello,

ARP is a level 3 (network) protocol. It is used to resolve IPs to MAC address. If the MAC address is not found in the ARP cache, then, the protocol broadcasts the entire network to find the corresponding MAC address (using layer 2).

It's something like "Who has 192.168.0.10?"... and the owner of the IP will respond "Me, my address is xxx:Xxxxx"

RARP is the other way... used mainly for bootp or something like that!

Hope it helps!

Yep thats a nice way of putting it Thx for the reply. Going to book this test verrrryyyy soon.. Just tidying up on the info
Thanks again!

Webmaster

I was kinda surprised to read:" "The techexams note that both these protocols exist at the data link layer 2 level."

Because when you look at my OSI model TechNotes, you'll see that they are listed in the Network layer:
www.techexams.net/technotes/ccna/osimodel.shtml
I couldn't find ARP in my Network+ OSI notes, but then noticed I did list them as Data Link layer protocols in the Network+ TCP/IP suite TechNotes.

Arp, rarp, etc are Internet layer protocol (see DoD model on bottom of my CCNA OSI notes). The DoD model layers doesn't 'exactly' map to the OSI layers, but the Internet layer is the Network layer in the OSI model.

- One could say they operate on OSI layer two because ARP messages are Ethernet frames. Also, ARP was originally designed for the DEC/Intel/Xerox 10Mbit Ethernet. Not specifically for IP, but specifically for 10Mbit Ethernet, to map 16-bit CHAOS addresses, 32-bit IP addresses
and 8-bit Xerox PUP addresses to the physical ethernet address.
- One could also say they reside on OSI layer three, because they are implemented by the network layer protocol.

Or OSI layer 2 because Cisco depicts so:


Or OSI layer 3 because Cisco says so:

Cisco.com wrote:

In order for devices to be able to communicate with each when they are not part of the same network, the 48-bit MAC address must be mapped to an IP address. Some of the Layer 3 protocols used to perform the mapping are:

•Address Resolution Protocol (ARP)
•Reverse ARP (RARP)
•Serial Line ARP (SLARP)
•Inverse ARP

When you look at these arguments, one would have to conclude that it is a layer 3 protocol that operates on layer 2 as well. It is the ARP module that actually constructs the ethernet frame (see rfc 826). It's implemented by the network protocol to send Ethernet frames in order to discover layer 3 addresses.

Someone just checked the Cisco courses for me, and in the Cisco academy, semester 1, they also list them at both layers (of the DoD model) and explictely mention ARP and RARP operate on both the DoD layer "Network Access" (which map to the physical and data link layer of the osi model) and the DoD layer "Internet" (maps to Network OSI layer). Though the picture and quote from Cisco above show even they disagree with themselves, as many other papers about ARP (many even don't mention it to avoid the discussion), this is because the OSI model is just that, a model, and even though it's developed to facilitate a modular approach, in reality network technologies and protocols don't always fit neatly within layers in the model.

For the Network+ exam, and this is the unwritten rule for other exams as well, 'if' you need to pick 'one' layer, pick the highest layer. If you have the ability to pick two answers you'll know what to do.

I'll update the Network+ to be reflect the CCNA notes, and will also add one or more lines explaining it.

agustinchernitsky

You are correct...

ARP uses layer 2 or DoD Internet level to access the MAC protocol to query for MAC addresses...

At least, I just checked Sybex CCNA book 5th ed and it states the same that you are saying...

good luck!

Danman32

The reason I would say ARP is layer 3 is because it is strictly for layer 3's benefit. Then again, one could say it is strictly for layer 2's benefit so it can connect to the layer 3 address.

ARGGG! This is like the tootsie pop commercial in trying to find how many licks to get to the center. Owl goes 'one, two, three <crunch>. Three'