> >At 10:57 PM 12/13/01 -0500, Chuck Larrieu wrote: >> >I once had an interesting, if heated argument with someone off list about >> >this. IIRC, I was told by that person that Cisco, in its current CCNP >study >> >materials, is saying just that - that something operates at the OSI layer >> >above which it functions. I.e. if a routing protocol uses an IP protocol >> >number, then it is operating at transport layer. Since BGP uses TCP port >> >179, it is operating at the session layer, along with RIP, which uses UDP >> >port 520. ( BTW, I have also read in a reputable source that UDP is >> >application layer because it is not reliable, and therefore cannot be >> >transport layer, and there is no place else it really fits ) > >Chuck, > >This is obviously nonsense, as I know that you know. I'm not criticizing >you, since you are quoting someone else, but this was a quote that should >have been routed directly to the null interface! ;-) > >> > >> >I recognize that Cisco just LOVES the OSI model in the lower level >> >certifications, but the fact is that in terms of how things work it is >crap, >> >and tends to cause more confusion and add no value. > >I disagree. I think the OSI model adds a lot of value for understanding the >functions of a protocol. It helps one understand what types of services a >protocol provides and what services it uses from the layer below.
Priscilla, I agree the layering concepts, not the layers themselves, are what is important. For whatever reason, however, a great many sources, including Cisco, completely ignore the architecturally critical difference between services and protocols. Your comment above, in a way, is that of an expert that has internalized the idea of service provision and service use, but most "OSI" discussions focus completely on protocol exchange. > >> > >> >Every vendor of content switches is calling them layer 4-7 switches. what >> >kind of crap is that? > >Switching of messages happens at all layers. That's the point of >networking! But the methods for doing it and the data used to do it differs >with each layer. > >Routing protocols are in the management and control side of the network >layer. They allow routers to learn how to switch packets based on >network-layer addresses. > >People get themselves in trouble when they characterize the layer that a >protocol works at by which protocols run below it and the number of >protocols that run below it. Routing protocols are not the only weird ones. >NetBIOS is a session-layer protocol, for example, but in a NetBEUI >implementation, it runs above LLC. That's doesn't change which OSI layer it >fits into best. > >Consider ISDN. ISDN has three layers. Running above ISDN may be the >Point-to-Point Protocol (PPP), which is usually considered a >data-link-layer protocol, although it has four layers of its own. Its top >layer provides a set of Network Control Protocols (NCPs) that are used to >establish and configure upper-layer protocols such as IP and IPX. Trying to >force all these layers into seven layers, especially when you need to >anchor IP at Layer 3, because you know it's a network-layer protocol, can >lead to frustration. It's best to just consider what services ISDN and PPP >offers and how they are used in typical networks, and stuff them into the >data-link layer. Especially with ISDN, the differentiation among management/control and user planes is important. The D channel stack, of course, is control oriented (I shall, with lordly disdain, ignore X.25 over LAP-D and V.120). Current practice tends to blur the distinction between control (host-to-network) and management (intranetwork), although I still find that useful. NCP, as of course you know, are control plane protocols for PPP, not necessarily seen at the data link service interface. In the updated OSI Internal Organization of the Network Layer, NCP also could be considered Subnetwork Dependent Access Protocols. > >With routing protocols, the important thing is that when you configure and >troubleshoot them, you aren't going to spend too much time considering >transport or application-layer issues. You aren't going to analyze sequence >numbers, ACKs, retransmissions, etc. You are going to focus on >network-layer issues such as addressing, forwarding, routing, router >configs, VLSM, classful versus classless, IP subnet zero, etc. Users are test programs for user applications. User applications are test programs for upper layer protocols. Upper layer protocols are test programs for the network layer, which is interesting. Protocols below the network layers are its slaves, unruly slaves at times. > >This is another one of those issues that is simply not worth debating. >Routing protocols clearly work at the network layer. I said all this much >better the last time this came up. ;-) See the archives. > >Priscilla > >> >I dare anyone to justify switching as a layer 5 or a layer 6 activity. Yet >> >there it is. Also, to judge from what content switches do, the marketers >are >> >saying the OSI layer 7 is user application, not a service application, >> >something Howard takes great pain to differentiate in his writings on the >> >subject, again IIRC. >> > >> >TCP/IP is NOT OSI compliant, never has been, never will be. OSI is a >> >reference model, and not necessarily related to anything in real life. >> > >> >End of rant. >> > >> >Chuck >> > >> >-----Original Message----- >> >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of >> >Jose Luis De Abreu >> >Sent: Thursday, December 13, 2001 12:25 PM >> >To: [EMAIL PROTECTED] >> >Subject: Routing protocols [7:29139] >> > >> > >> >Just an open question ? >> > >> >We read, learn and teach Routing protocols are at the >> >NETWORK layer of the famous OSI model... >> > >> >But they have PROTOCOLS NUMBERS - TRANSPORT LAYER(such >> >as IGRP protocol 9, EIGRP protocol 88 and OSPF >> >protocol 89)and APPLICATION PORTS values - APPLICATION >> >LAYER (RIP uses port 520 and BGP4 uses port 179) >> >indicating they work in the upper layers and not in >> >the network layer, although the result is shown int >> >the NETWORK layer... >> > >> >So may question is... >> > >> >Do they really operate at LAYER 3 ? >> > >> >Warm regards, >> > >> >Jose Luis De Abreu >> > > > >________________________ > >Priscilla Oppenheimer >http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=29228&t=29139 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
