What about when the router itself is generating the ARP? I need to find more details
about how the ARP packet is constructed. I see a little bit about it in my CIT study
guide, but I'd like more details, specifically about the protocol field and its
options.
If there were no default encapsulation type, I'm thinking that the router would not be
able to construct ARP packets. But then we would all just have to learn to add
something like "encapsulation arpa" to our ethernet configs.
I still don't really see why there *has* to be a default, except that it might lessen
the configuration burden on us.
>
> I am also confused about the question itself.
>
> The basic default behavior of Cisco routers is to try to identify the
> protocol type of a frame, and send it either to the bridging code or
> the router code for that protocol on that interface. If the
> protocol type is not supported, drop the frame.
>
> I don't know this, but I can reasonably assume that IP protocol
> identification is hard-coded for the Ethernet II IP Ethertype and for
> 802.2 IP. IP might be treated as a special case, of which the router
> is agnostic about encapsulation Still, the encapsulation should be
> meaningful if and only if appropriate routing or bridging is enabled
> for the interface.
>
> ARP has a protocol type field so that wouldn't seem like the reason.
>
>
> >Okay, I have a guess...a total W.A.G., and I wasn't able to back it
> >up with some quick research, but here it is:
> >
> >The answer to this has something to do with ARP packets. My guess
> >is that they assume the presence of Ethernet_II frames when
> >constructing ARP packets and these would be inoperable if some other
> >ethernet frame were being used.
> >
> >That would explain why the default *had* to be Ethernet_II. If ARP
> >breaks, IP over ethernet breaks.
> >
> >Am I right?? I'm going to keep digging to see if I can find some
> >more details about this. I may be chasing down the wrong street.
> >Let me know if I'm even close! :-)
> >
> >John
> >
> >>
> >> OK, Leigh Anne, you're just going to have to come out and tell us what you
> >> are getting at. The suspense is killing me. &;-)
> >>
> >> The only time I've ever configured an Ethernet encapsulation, it has been
> >> part of the ipx network command. As we know, Novell mucked things up and
> >> supports four frame types, so being able to configure the frame type is
> >> necessary for IPX. A unique feature of IPX is that you can configure
> >> multiple networks on a single segment. Each of them must have a different
> >> encapsulation. In fact that is how you support networks with devices
> >> configured for different encapsulations.
> >>
> >> I don't even know that you can configure the encapsulation for IP on
> >> Ethernet on a router. Can you? Or is that what you're getting at. IP
> >> doesn't care.
> >>
> >> With IP, 99% of the world uses Ethernet V2 (dest, src, EtherType). I just
> >> tried to change it on my PC and I couldn't, although I think I have seen
> >> that capability on other PCs. But my guess is that if I did change it, the
> >> router could still handle it.
> >>
> >> Priscilla
> >>
> >> At 12:33 PM 2/7/01, Tony van Ree wrote:
> >> >Hi,
> >> >
> >> >I understood it to tell me that there is a common method used by a number
> >> >of manufacturers and protocols. Some other companies and protocols had
> >> >made some changes. The default was used as it was the most
> >> >common. Ethernet_II had been around for quite a while before the 802.3
> >> >and almost all devices manufacturers ethernet cards and the like could
> >> >handle Ethernet_II but not necessarily 802.3.
> >> >
> >> >Maybe I mis understood.
> >> >
> >> >Teunis
> >> >
> >> >
> >> >On Tuesday, February 06, 2001 at 05:36:32 PM, [EMAIL PROTECTED] wrote:
> >> >
> >> > > I did read Priscilla's post. She addressed the issue of WHY
> >>Ethernet_II is
> >> > > the default frame type selected for IP, but didn't examine why IP
> >> > requires a
> >> > > default frame type in the first place. IPX uses a default frame type
> >> > > because different Ethernet encapsulations are not able to co-exist
> >> > within an
> >> > > IPX network -- however different Ethernet encapsulations
> >>(Ethernet_II and
> >> > > Ethernet 802.3) ARE able to co-exist within an IP network. As such,
> >> > what is
> >> > > the importance of a default Ethernet encapsulation for IP?
> >> > >
> >> > > That's what I've been challenging John to think about. Once
> >>he understands
> >> > > where the default Ethernet encapsulation comes into play, he
> >>could answer
> >> > > his question as to whether there "would there be a good reason
> >>to change to
> >> > > a
> >> > > different frame type, or would we only benefit from a
> >>different frame type
> >> > > in a non-IP environment or mixed environment".
> > > > >
> >> > >
> >> > >
> >> > >
> >> > > -----Original Message-----
> >> > > From: Tony van Ree [mailto:[EMAIL PROTECTED]]
> >> > > Sent: February 6, 2001 5:06 PM
> >> > > To: [EMAIL PROTECTED]; John Neiberger; [EMAIL PROTECTED]
> >> > > Subject: RE: Another 802.3 and Ethernet Question
> >> > >
> >> > >
> >> > > Hi,
> >> > >
> >> > > I'm sorry I did not cover the rest of the ethernat frame
> >>types. This was
> >> > > covered earlier this week. Priscilla covered it really well
> >>in one of her
> >> > > replys on a similar question.
> >> > >
> >> > > Teunis
> >> > >
> >> > >
> >> > > On Tuesday, February 06, 2001 at 04:55:01 PM, [EMAIL PROTECTED] wrote:
> >> > >
> >> > > > Yes, with respect to IPX, that's correct--and that answers my first
> >> > > > question.
> >> > > >
> >> > > > My second question asked about what was the purpose of a
> >>default Ethernet
> >> > > > frame type for use with IP. Using IPX as an analogy, does a
> >>router only
> >> > > > route Ethernet_II frames if no Ethernet frame type has been specified?
> >> > > Does
> >> > > > a router drop IEEE 802.3 frames by default? To route IEEE
> >>802.3 frames,
> >> > > is
> >> > > > any additional configuration required?
> >> > > >
> >> > > > And with that, we're lead back to John's original question:
> >>What is the
> >> > > > purpose of a default Ethernet frame type for IP?
> >> > > >
> >> > > > -----Original Message-----
> >> > > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> >> > > > Tony van Ree
> >> > > > Sent: February 6, 2001 2:51 PM
> >> > > > To: Leigh Anne Chisholm; John Neiberger; [EMAIL PROTECTED]
> >> > > > Cc: Priscilla Oppenheimer
> >> > > > Subject: RE: Another 802.3 and Ethernet Question
> >> > > >
> >> > > >
> >> > > > Hi,
> >> > > >
> >> > > > Those not specified by the router are either routed by the server or
> >> > > produce
> >> > > > IPX protol errors and are dropped.
> >> > > >
> >> > > > It is important not to have the various frame types set on
> >>the servers or
> >> > > > service advertisers. If for example you are normally using
> >>Novell-Ether
> >> > > > (802.3) and you put in a server using Netware 4.x running SAP (802.2).
> >> > > Now
> >> > > > when you put in the first server you configure both the SAP and Novell
> >> > > Ether
> >> > > > in the server. You have 802.3 (Novell-ether) configured in
> >>the router.
> >> > > > Pull out the original server and you have no network. Othen
> >>you will lose
> >> > > > half of your local clients.
> >> > > >
> >> > > > Have lose networks and or frame types can also create some horrible
> >> > little
> >> > > > routing loops and unwanted traffic. SAP's, RIP updates etc.
> >> > > >
> >> > > > Let the router route and servers serve.
> >> > > >
> >> > > > Another one that sometimes grabs you.
> >> > > >
> >> > > > Teunis,
> >> > > > Hobart, Tasmania
> >> > > > Australia
> >> > >
> >> > >
> >> > >
> >> >
> >> >
> >> >--
> >> >www.tasmail.com
> >> >
> >> >
> >> >_________________________________
> >> >FAQ, list archives, and subscription info:
> >> >http://www.groupstudy.com/list/cisco.html
> >> >Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
> >>
> >>
> >> ________________________
> >>
> >> Priscilla Oppenheimer
> >> http://www.priscilla.com
> >>
> >> _________________________________
> >> FAQ, list archives, and subscription info:
> >>http://www.groupstudy.com/list/cisco.html
> >> Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
> >
> >
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> >_________________________________
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> _________________________________
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