Ezra and Inayath,
Especially with protocols like HSRP or ARP, the discussion about "which layer they fit into" can quickly grow to a holy war. The fact is that IP-based protocols have never truly respected strict layering, not in TCP/IP architecture nor in ISO OSI model, and hence there are many protocols that truly cross the boundaries of multiple layers.
Instead of first asking "what layer HSRP operates on?", we should ask: "what is the rule by which we can decide on what layer a protocol operates on?" Indeed, what is often unclear is the criterion itself we should be using to decide what layer does a protocol operate on.
Some people propose that the defining property is the set of protocols and layers our protocol needs itself in order to operate, because in that case, the protocol in question is directly above the required layers. That puts IP quite correctly into the Layer3 (because it needs services of Layer1 and Layer2 to operate), TCP and UDP into Layer4 (because they expect Layer1-3 services to be available), and so on. However, this criterion also makes, say, RIP protocol to be at least Layer5 or above because it needs Layer1-Layer4 to be working in order for RIP to work itself (RIP runs over UDP). The same goes for BGP as it runs over TCP. Now we feel here that considering RIP or BGP to be a true application protocol is, frankly, ridiculous. Even OSPF is sometimes considered a Layer4 protocol because it runs over IP, but come on! We expect segmentation, reassembly, connection-oriented and connectionless, reliable and unreliable delivery of data as the service set provided by Layer4 - this is its purpose - what has OSPF to do with this?
Therefore, I personally propose a different view - the defining property is the set of services the protocol provides itself, and the user of these services. Let us take, for example, routing protocols into a closer view. Regardless of what protocols they require in order to operate themselves, the primary role of routing protocols is to supply the Layer3, in this case, IP, with vital information about how it should operate. In other words, routing protocols provide control services to Layer3 and their entire focus is Layer3 operations from the control point of view, so this would make all routing protocol to be considered as Layer3 control/management protocols. Note, for example, that it is not necessary for an IP routing protocol to necessarily run over IP packets itself. OSPF, EIGRP, RIP, BGP do. IS-IS does not - its messages are placed directly into Layer2 frames - but that does not make it any less proper. You see, you can have your routing table perfectly complete both with IP-based OSPF and Layer2-encapsulated IS-IS. I even heard a question to EIGRP developers on Live! 2013 in London if they are considering bypassing IP encapsulation and moving EIGRP down to direct Layer2 encapsulation Would that make EIGRP to be suddenly a different layer protocol? Hardly - it would still serve the same layer and provide the same set of services.
And again, IS-IS, a well known Layer3 routing protocol as we know it, running in a TRILL or FabricPath network - is it still a Layer3 routing protocol? I do not believe so - what it does here is computing shortest paths between TRILL RBridges or FP switches so that switching over shortest paths can be performed, but here, IS-IS helps to provide Layer2 switching functions with relevant control information. Did the IS-IS change in any way? No, it did not, just the intended user of its services did. Here, it is Layer2 who requires its services to do its own job.
In the same way of thinking, I treat ARP as a Layer3 adaptation control protocol that allows Layer3, in this case, IP, to adapt its particular operation to a specific Layer2 - Ethernet - framing, allowing proper encapsulation of IP packets into Ethernet frames regarding the addressing. ARP is used by IP, not by Ethernet itself, and the service it provides is mapping IPs to MACs. The ARP table is maintained by IP protocol, not by Ethernet, and a different Layer3 protocol, say, IPv6, uses a different approach to create IP-to-MAC mappings, so ARP is quite closely tied to IPv4.
With HSRP, we should first think about whether the resolution of the default gateway IP address is a Layer3 or Layer2 function. Clearly, selecting a proper gateway is a matter of Layer3 routing, so this is a Layer3 operation. Resolving the IP address of the default gateway into an appropriate Layer2 address is a matter of Layer3 adaptation layer for Layer2, so again a Layer3 specific operation although it does interface directly with Layer2 operations. HSRP merely makes sure that the IP address of the default gateway and the associated MAC address are transparently handled by a particular router in a standby group so that there is always exactly one router responsible for this vIP/vMAC. From my point of view, therefore, HSRP is a Layer3 control protocol that virtualizes the default gateway IP/MAC but it still concerns purely with IP and adaptation of IP to MAC.
Wow, I did not intend to write such a lengthy post in the beginning... I guess this is a neverending topic.
My two cents...
Best regards,
Peter
