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ARP and ICMP Which layers??

Oct 25, 2011 9:13 AM

biplab 16 posts since
Jun 8, 2011

hi everyone,

          i have read a few books including the cisco ccna cetification guide and the 31 days before ccna. I found that these books have stated ARP and ICMP as a layer 2 protocol for OSI model (i.e. data link layer) and a layer 2 for TCP/IP model too (i.e. Internet layer). But if the protocol were to tally shouldn't the ARP and ICMP be on the first layer of the TCP/IP model since the first layer links directly to the data link and physical layer of the OSI model and data link defines the ARP and ICMP??

 

thank you,

 

Biplab

  • Helder Neves 346 posts since
    Jun 17, 2011
    Currently Being Moderated
    1. Oct 25, 2011 9:17 AM (in response to biplab)
    Re: ARP and ICMP Which layers??

    ARP and ICMP is layer 3 - Network Layer.

     

    Layer 2 is Ethernet, PPP, HDLC, DSL, Frames, Network Switching, MAC address ...

     

     

    Regards

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    3. Oct 25, 2011 9:23 AM (in response to biplab)
    Re: ARP and ICMP Which layers??

    ICMP is a Layer3 protocol.  Not sure who said it was Layer2.

     

    ARP, on the other hand, while I might be willing to say it's Layer 2.5, I really have a hard time calling it a Layer3 protocol because there's nothing about specific or hierarchical addresses to it at all!

     

    It's a broadcast sent AT LAYER2 in order to figure out who has a Layer3 IP address.  So it SUPPORTS layer 3, but that doesn't make it a Layer3 protocol.  

     

    (Although it's funny this question came up because in the CCIE Security meeting group I'm in this week, someone pointed out an article on Cisco's web site that called ARP a Layer3 protocol and so we all had been debating that and laughing about it.)

     

    HTH,

     

    Scott

  • Helder Neves 346 posts since
    Jun 17, 2011

    Hmm im a bit confused now lol. At now i think it is a layer 3, i have read now on books it is sended to discover MAC ADDRESSES, so it can be called a layer 2 protocol, other reason to be a layer 2 is, ARP send a broadcast frame so is layer 2 because layer 3 devices dont forward broadcasts !

     

     

    Whats your opinion?

     

    Regards

  • Garri 111 posts since
    Jun 20, 2011
    Currently Being Moderated
    5. Oct 25, 2011 11:44 PM (in response to biplab)
    Re: ARP and ICMP Which layers??

    Hi biplab,

     

    ARP runs on top of layer 2 protocol, i.e. using services provided by data-link protocols. ICMP runs on top of layer 3 protocol.

     

    For example, ICMP uses IP protocol to delivery ICMP datagram to remote host in remote, non-local network. ARP uses Ethernet protocol to flood out question "Who is x.x.x.x?' over switched network.

     

    Kind Regards,

    Garry

     

    Message was edited by: Garry

  • Garri 111 posts since
    Jun 20, 2011
    Currently Being Moderated
    7. Oct 25, 2011 10:07 AM (in response to biplab)
    Re: ARP and ICMP Which layers??

    biplab, you shouldn't consider ARP as the datalink protocol, as it doesn't transport the layer 3 protocols. Rather, as Scott said it is a function (component) of the layer 3 protocols, such as IP, to determine a hardware address of a host. ARP uses the layer 2 protocols to complete this function.

     

    HTH, Garry     

    ,

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    8. Oct 25, 2011 10:08 AM (in response to Helder Neves)
    Re: ARP and ICMP Which layers??

    If you're on a Layer2 network, why would you need to forward broadcast across a Layer3 boundary?

     

    Yes, it's needing to figure out a MAC (the reply).  And it knows the IP address.  but in the FRAME HEADER (note, not an IP header) there is no hierarchical address information.   That's in the content/data.

     

    Like Garry says it just asks the question of "who has x.x.x.x?"...

     

    ICMP though has information that directs it (potentially across Layer3 boundaries) to a specific layer 3 address.  ARP will not cross any Layer3 boundary and has no reference to reach a specific layer 3 address.

     

    All the joys. 

     

    Scott

  • Garri 111 posts since
    Jun 20, 2011
    Currently Being Moderated
    10. Oct 25, 2011 10:24 AM (in response to biplab)
    Re: ARP and ICMP Which layers??

    There is a video on CNL describing both.

    https://learningnetwork.cisco.com/docs/DOC-1642

     

    Kind Regards,

    Garry

  • Bikas Pandey 44 posts since
    Jan 4, 2011
    Currently Being Moderated
    11. Oct 25, 2011 11:55 AM (in response to biplab)
    Re: ARP and ICMP Which layers??

    ARP is layer two and ICMP is layer 3 protocol

  • Elvin Arias 1,837 posts since
    Mar 12, 2010
    Currently Being Moderated
    12. Oct 25, 2011 3:55 PM (in response to biplab)
    Re: ARP and ICMP Which layers??

    ARP is layer two and ICMP is layer 3. Good discussion .

     

    Elvin

  • Paul Stewart  -  CCIE Security, CCSI 6,966 posts since
    Jul 18, 2008
    Currently Being Moderated
    13. Oct 25, 2011 7:11 PM (in response to Elvin Arias)
    Re: ARP and ICMP Which layers??

    The OSI model is just that, a model. Therefore, you will find inconsistent opinions. In my mind ARP is clearly a layer two protocol. It enables IP over Ethernet. ICMP is a layer 3 protocol that depends on IP. There is more gray area in calling ICMP layer a layer 3 protocol than calling arp a layer 2 protocol. At least in my opinion.

  • Garri 111 posts since
    Jun 20, 2011
    Currently Being Moderated
    14. Oct 26, 2011 12:28 AM (in response to biplab)
    Re: ARP and ICMP Which layers??

    I think we should classify a protocols running directly on particular layer of OSI model (using the transport services of  the protocols running at lower layers and providing the transport services for the protocols running at upper layer), and the protocols running between layers of the OSI model (using the transport services of  the protocols running at lower layers, not providing transport services to upper layer protocols, but specific supplementary services).

     

    For example,

    - Ethernet and PPP runs directly on a layer 2 of the OSI model (for instance, Ethernet using the transport services of layer 1 electrical signal transmission and providing the transport services for layer 3 IP and layer 2.5 ARP).

    - ARP runs on top of a layer 2 (for instance, ARP using the transport services of the layer 2 Ethernet and providing a supplementary services for layer 3 IP, not transport services).

    - IP and IPX runs directly on a layer 3 of the OSI model (for instance, IP using the transport services of layer 2 Ethernet protocol and providing the transport services for the layer 4 TCP or UDP, or maybe for the layer 3.5 ARP protocol). 3.5 - for simplicity :).

    - ICMP runs on top of a layer 3 (for instance, using the transport services of layer 3 IP and providing the supplementary services for applications).

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