Zsombor Papp wrote: > > At 10:21 PM 6/26/2003 +0000, Priscilla Oppenheimer wrote: > >Oh. Your message didn't say anything about IPX. > > Right. I was talking about layer 2 encapsulations. I thought > that the fact > that *Cisco IOS* supports this configuration only for IPX is > irrelevant.
I don't think it's irrelevant that Cisco IOS (and most operating systems) only support configuring the Ethernet encapsulation for IPX. Newbies think you can configure the Ethernet encapsulation in a generic way and for multiple protocols. You can't, and you wouldn't want to anyway if you want to interoperate with any other device. You can configure the type of VLAN tagging that should be used and then there's the IPX anomoly that we have been living with since 1981. With Cisco IOS, the fact that you can enter a generic "encapsulation" command on serial interfaces that affects all packets and that you can't do this on Ethernet is an important distinction that has caused confusion for numerous people on this list over the years. I thought it was worth further discussion, though not this much discussion! :-) > Maybe I was wrong, see below. > > >Ethernet encapsulation depends on the payload. > > I am a bit surprised by this statement. Are you saying that the > Ethernet > specifications mandate the usage of, say, Ethernet II > encapsulation if you > want to transport IP packets? No. Ethernet specs don't say what the payload will be. > Frankly, I have never read the > Ethernet > specifications, Well, I have for what it's worth, which is not a whole lot. :-) I've read every Ethernet spec since Bob Metcalf's original memo from 1973. > but I thought that *in theory*, you can pretty > much > transport any payload in any Ethernet encapsulation, it's just > *usually* > not done. Am I mistaken? No. You're not mistaken. I think it's interesting and relevant that IP always uses Ethernet II on essentially every modern operating system, even though Ethernet II is old. I think it's interesting that other protocol developers have made other choices for the encapsulation. I also know that it's an area of confusion for 1000s of students of networking, especially Cisco certification candidates, even though Cisco doesn't make you learn protocols. But I'll help people learn protocols and you can't stop me! :-) Priscilla > If so, can you point me to some > documents that > would enlighten me? (Seriously.) > > The fact that something is "not configurable" doesn't prove > anything > outside of the scope of the IOS implementation. > > As a side question, do you think that TCP must run over IP? :) > > Thanks, > > Zsombor > > > IP uses Ethernet II, CDP uses > >SNAP, etc. With the exception of IPX, it's not configurable. > (Actually ARP > >is configurable too for historical reasons. Long story that I > can't get into > >now). > > > >That's not like encapsulation on a serial interface that > doesn't care about > >the upper layer. > > > >Think about the IOS software. It has to know what it is > encapsulating on > >Ethernet. It doesn't on a serial interface. > > > >That was what I meant by the behavior being different. > > > >Priscilla > > > > > > > > If you mean that the original question was very Frame Relay > and > > > Cisco IOS > > > implementation specific, and compared to that my answer was > too > > > generic and > > > theoretical, then you are probably right. :) > > > > > > Thanks, > > > > > > Zsombor > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=71498&t=71421 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
