1 2 3 Previous Next 81 Replies Latest reply: Aug 20, 2014 2:43 AM by AICCOID22122009 RSS

    ARP and ICMP Which layers??

    biplab

      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

        • 1. Re: ARP and ICMP Which layers??
          Helder Neves

          ARP and ICMP is layer 3 - Network Layer.

           

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

           

           

          Regards

          • 2. Re: ARP and ICMP Which layers??
            biplab

            Thanks, Helder i was a bit confused on this one i will set that to my mind.

             

            Regards,

            Biplab

            • 3. Re: ARP and ICMP Which layers??
              Scott Morris - CCDE/4xCCIE/2xJNCIE

              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

              • 4. Re: ARP and ICMP Which layers??
                Helder Neves

                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

                • 5. Re: ARP and ICMP Which layers??
                  Garri

                  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

                  • 6. Re: ARP and ICMP Which layers??
                    biplab
                    Internet Layer
                    • IP (IPv4IPv6)
                    • ICMP
                    • ICMPv6
                    • IGMP
                    • IPsec
                    Link Layer
                    • ARP/InARP
                    • NDP
                    • OSPF
                    • Tunnels (L2TP)
                    • PPP
                    • Media Access Control (EthernetDSLISDNFDDI)

                    this is what i found in wiki. And where did the data link layer came from on TCP/IP stack. They should either define it as a network access layer or  change the whole stack to OSI reference?? Confused...confused....So if i tally the arp to TCP/IP, should not that be in the  layer one of the TCP/IP ?? but the book here says it's on layer two of TCP/IP which is linked to the  third layer of the OSI model when referenced??

                     

                    regards,

                    Biplab

                    • 7. Re: ARP and ICMP Which layers??
                      Garri

                      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     

                      ,

                      • 8. Re: ARP and ICMP Which layers??
                        Scott Morris - CCDE/4xCCIE/2xJNCIE

                        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

                        • 9. Re: ARP and ICMP Which layers??
                          biplab

                          thank you everyone for your help.

                           

                          regards,

                          biplab

                          • 10. Re: ARP and ICMP Which layers??
                            Garri

                            There is a video on CNL describing both.

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

                             

                            Kind Regards,

                            Garry

                            • 11. Re: ARP and ICMP Which layers??
                              Bikas Pandey

                              ARP is layer two and ICMP is layer 3 protocol

                              • 12. Re: ARP and ICMP Which layers??
                                Elvin Arias

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

                                 

                                Elvin

                                • 13. Re: ARP and ICMP Which layers??
                                  Paul Stewart  -  CCIE Security

                                  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.

                                  • 14. Re: ARP and ICMP Which layers??
                                    Garri

                                    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).

                                    1 2 3 Previous Next