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



        I don't disagree with your logic.  However, there are instances where protocols don't necessarily fulfill the other requirements of a layer of the OSI model.  In some cases these are clear.  For example, IP and IPX are clearly layer 3 protocols.  They utilize the datalink layer for transport (I agree with that).  Arp also uses layer two for transport, but it clearly has no layer 3 headers (only layer 3 data), is not routable, and is an ethernet only concept.  People make arguments that it should be classified at layer 3.  I would argue that it doesn't fulfill the intent of layer three of the OSI model.  The OSI model was not built around the IP Protocol.  ICMP is also a gray example.  It does not have the features of a transport layer protocol, but clearly depends on IP as a transport mechanism.  When you get above the transport layer, the concept of encapsulation goes away, and thus some of the dependency on lower layers transporting upper layers (at least with in the upper three layers).  These are great conversations.  The bottom line is that the OSI model is just that--a model.  When applied, we won't all agree on how it relates.  However, these discussions clearly illustrate everyone's understanding of the protocols.  That's far more important that the interpretation of the intent of a layer in the model (a it's applicability to a protocol).

        • 16. Re: ARP and ICMP Which layers??
          Vijay Swaminathan

          Regarding Post 6, I really doubt the authenticity of this information.


          OSPF at Data Link Layer ?? I really doubt.

          • 17. Re: ARP and ICMP Which layers??

            Hi Vijay,


            I agree with you, OSPF work on top of OSI layer 3 protocol. IP protocol uses decimal 89 value to represent OSPF protocol.


            Kind Regards,


            • 18. Re: ARP and ICMP Which layers??

              A template, presented in wiki actually the TCP/IP template, not OSI model. TCP/IP Link layer defined as '

              the group of methods or protocols that only operate on a host's link'. OSPF included in TCP/IP Link layer because it runs on a host's link.


              More detailed information about TCP/IP Link layer definitions



              Kind Regards,


              • 19. Re: ARP and ICMP Which layers??

                Per RFC 792 & 1122, ICMP is one of the core protocols of the IP protocol suite, which includes IP, ICMP and IGMP.  ICMP, uses the basic support of IP as if it were a higher  level protocol.  However, ICMP is actually an integral part of IP, and  must be implemented by every IP module.  Per RFC 1122, ARP is part of the Link Layer.


                Hope this helps.




                • 20. Re: ARP and ICMP Which layers??

                  I think ARP is layer 3 because i can't "show arp" full information on layer 2 deviece but i can "show arp" full information on layer 3 deviece.

                  • 21. Re: ARP and ICMP Which layers??

                    I like to think of ARP as a subnetwork convergence protocol. As you said, layer 2.5. It has features of both layers, in a way. I see no reason to shoe-horn it into the pared-down OSI model that Cisco teaches. Well, other than the fact that they might ask you about it on a test. 

                    • 22. Re: ARP and ICMP Which layers??

                      This question would be an example of Cisco trying to shoe-horn certain concepts into their simplified OSI model and I don't think it really fits well. ARP isn't an OSI protocol, but it fills the role of what the OSI refers to as subnetwork dependent convergence facilities. The IETF would probably say it is layer 2.


                      ICMP rides on IP, so some people would say it is layer 3. I think this is again trying to shoe-horn a concept into a simplified model. ICMP is a layer management protocol that doesn't fit into the data plane. It is a control plane (or maybe management plane) function that legitimately should reside in a parallel stack next to the data plane stack.


                      Sorry if this just adds to the confusion.

                      • 23. Re: ARP and ICMP Which layers??

                        In exam you should answer ARP is layer3.

                        • 24. Re: ARP and ICMP Which layers??

                          ARP is a layer 2 (As it finds the IP address using MAC address {broadcast} )

                          ICMP is a layer 3 (Used to reach and check the path using IP address)

                          • 25. Re: ARP and ICMP Which layers??
                            Joshua Johnson - CCNP R&S

                            Scott Morris - CCDE/4xCCIE/2xJNCIE wrote:


                            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.)





                            haha, good thing you weren't drinking or you would have been fist fighting about it

                            • 26. Re: ARP and ICMP Which layers??
                              Joshua Johnson - CCNP R&S

                              Sounds like a good start for a Cisco movie.  Scott involved in a royal rumble over ARP. 

                              • 27. Re: ARP and ICMP Which layers??

                                In Kevin Wallace's Network+ Cert Guide (9780789748218), he clearly states ICMP is a layer 4 protocol. Picture of text is attached.


                                Sorry if I added confusion, but I, myself, am confused.ICMP.jpg

                                • 28. Re: ARP and ICMP Which layers??

                                  These type of questions are asked all the time on these forums and they always get a lot of attention. The reason is that there are passionate people on both sides. ARP is layer 3 vs layer 2. ICMP is layer 3 vs layer 4. Both sides have completely valid arguments to defend their positions. The truth is that some protocols don't fit perfectly into a single layer. The only reason it matters is when you're asked about it on a test. For those occasions simply understand what the tester's position is and give that answer. That doesn't mean it's the right answer, it's just their answer.


                                  No matter who takes what position there will always be someone who disagrees. Just remember it's quite possible that you're both right and in the end it really doesn't matter.


                                  • 29. Re: ARP and ICMP Which layers??

                                    That is incorrect. ICMP is implemented as part of the network layer. I think many get confused because it is (necessarily) encapsulated within IP packets, so they assume that it must reside at the layer above IP. Unfortunately, the simplified OSI model that Cisco likes to teach does not fit here, and neither does it fit with ARP. There are many arguments about whether ARP is layer 2 or 3, when it is in fact neither if you want to fit it into OSI. It is a subnetwork convergence protocol that is a bridge of sorts between layers 2 and 3.


                                    But as Brad said just a few moments ago, most of this is actually irrelevant and you'll find no shortage of people who disagree about what goes where. In the end, as long as you know what the protocols do, it's not so important to know what layer they're at. Unless of course you get asked that question on an exam, in which case the right answer is always the Cisco answer.  :-)

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