My understanding of Ethernet having previously read RFC 1042 has been (as
Anthony pointed out), that a router that receives 802.3 frames on an
Ethernet interface will send out frames out using 802.3 encapsulation. A
router that receives Ethernet_II frames on an Ethernet interface will send
out frames using Ethernet_II encapsulation. If an end-system sends out
802.2 SNAP information within an 802.3 frame, would the router send this
information within an Ethernet_II frame encapsulation? I've always thought
that it wouldn't.
Which leads to the question - why a default frame type, if the default frame
type isn't used as an encapsulation frame type for creating Ethernet frames
received by an end-system?
Essentially, it's been my understanding that the default Ethernet_II frame
encapsulation has been used where a packet originates within the router
requiring a "first-time" Ethernet encapsultion. By that, I'm referring to
telnet packets originating within the router - they need to be encapsulated
in something as they go out an Ethernet interface. Alternately, packets
received from a serial interface, token-ring interface, or FDDI interface
that needs to be "popped-into" an Ethernet frame format would use the
Ethernet_II encapsulation method.
What benefit would it be to use an Ethernet 802.3 encapsulation rather than
an Ethernet_II encapsulation method? You don't really get any benefit in my
opinion. Ethernet_II has everything you need, and it's supported by all
Ethernet devices since the inception of Ethernet_II equipment.
It was just one of those things that made me go "hmm..." when I first
stumbled onto the question...
I think I think too much.
(-:
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
anthony kim
Sent: February 7, 2001 8:01 PM
To: [EMAIL PROTECTED]
Subject: Re: Another 802.3 and Ethernet Question
Hi,
I had to take a look at rfc 894 (ethernet) and rfc 1042 (ieee802)
from rfc 1042:
"
It is possible to use the Ethernet link level protocol [12] on the
same physical cable with the IEEE 802.3 link level protocol. A
computer interfaced to a physical cable used in this way could
potentially read both Ethernet and 802.3 packets from the network.
If a computer does read both types of packets, it must keep track
of which link protocol was used with each other computer on the
network and use the proper link protocol when sending packets.
One should note that in such an environment, link level broadcast
packets will not reach all the computers attached to the network,
but only those using the link level protocol used for the broadcast.
Since it must be assumed that most computers will read and send
using only one type of link protocol, it is recommended that if such an
environment (a network with both link protocols) is necessary, an
IP gateway be used as if there were two distinct networks.
"
...
then i read rfc1122:
"
Every Internet host connected to a 10Mbps Ethernet cable:
o MUST be able to send and receive packets using RFC-894
encapsulation;
o SHOULD be able to receive RFC-1042 packets, intermixed
with RFC-894 packets; and
o MAY be able to send packets using RFC-1042
encapsulation.
An Internet host that implements sending both the RFC-894 and
the RFC-1042 encapsulations MUST provide a configuration
switch to select which is sent, and this switch MUST default to RFC-
894.
"
On Tue, Feb 06, 2001 at 10:34:21PM -0800, John Neiberger wrote:
>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]
>
>
>Find the best deals on the web at AltaVista Shopping!
>http://www.shopping.altavista.com
>
>_________________________________
>FAQ, list archives, and subscription info:
http://www.groupstudy.com/list/cisco.html
>Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com
_________________________________
FAQ, list archives, and subscription info:
http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
_________________________________
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
