Ethernet Frame (revisited for clarification)
this post(s) was posted a couple of days back and just wanted some more list memebers to see if this correct before we take this as gospel truth. --- Neil Desai <[EMAIL PROTECTED]> wrote: > To my knowledge serial links don't have a MAC > address. Since most of them > are either a point-to-point or point-to-multipoint > there are some other type > of mappings. If a serial port needs a MAC address it > usuall uses one from > another interface that has one (i.e. ethernet). > Neil > ""Martinez, Carlos"" <[EMAIL PROTECTED]> > wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > hello all, > > > > I had somebody ask me what the source mac address > would be on a frame sent > > across a serial link connected by to two routers, > > for example: Host A sends a packet to Host B, > which is on the other side > of > > the wan link. what would Host B see and what where > would he send his reply > > to.(the local router or Host A or what) > > > > thanks in advance > > > > _ > > 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] __ Do You Yahoo!? Yahoo! Shopping - Thousands of Stores. Millions of Products. http://shopping.yahoo.com/ _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Ethernet Frame (revisited for clarification)
If you look at the frame format for any LAN protocol you will see where the Destination and Source MAC address are. You will not see these in any of the WAN frame formats. I looked on CCO for more information to clarify this but was unsuccessful. What I did look at was the frame formats for different WAN protocols. Here some links to show you what I am talking about: SDLC http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/sdlcetc.htm#xtocid2 49413 Frame Relay http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm#41825 X.25 http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/x25.htm#xtocid12273 10 Ethernet http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ethernet.htm#xtocid 118335 Token Ring http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/tokenrng.htm#xtocid 73166 FDDI http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/fddi.htm#xtocid1028 610 If you look at http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introwan.htm you will see that where the WAN technologies lay in the OSI model and hopefully this will also explain why serial lines don't have MAC addresses. The reason that LAN protocols have a MAC address and WAN's don't is because LAN's are contention based where WAN's are always full-duplex. Even though layer 2 switching has been around for LAN's for a few years now the protocols have stayed the same for backwards compatibility. Neil "John Green" <[EMAIL PROTECTED]> wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > this post(s) was posted a couple of days back and just > wanted some more list memebers to see if this correct > before we take this as gospel truth. > --- > Neil Desai <[EMAIL PROTECTED]> wrote: > > To my knowledge serial links don't have a MAC > > address. Since most of them > > are either a point-to-point or point-to-multipoint > > there are some other type > > of mappings. If a serial port needs a MAC address it > > usuall uses one from > > another interface that has one (i.e. ethernet). > > Neil > > ""Martinez, Carlos"" <[EMAIL PROTECTED]> > > wrote in message > > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > hello all, > > > > > > I had somebody ask me what the source mac address > > would be on a frame sent > > > across a serial link connected by to two routers, > > > for example: Host A sends a packet to Host B, > > which is on the other side > > of > > > the wan link. what would Host B see and what where > > would he send his reply > > > to.(the local router or Host A or what) > > > > > > thanks in advance > > > > > > _ > > > 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] > > > __ > Do You Yahoo!? > Yahoo! Shopping - Thousands of Stores. Millions of Products. > http://shopping.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]
Re: Ethernet Frame (revisited for clarification)
Of course it's correct, but it's not really relevant to the question, which was asking about addressing end-to-end. Presumably Host A and Host B are on LANs, so think the problem the rest of the way through.. Priscilla At 09:47 PM 11/23/00, John Green wrote: >this post(s) was posted a couple of days back and just >wanted some more list memebers to see if this correct >before we take this as gospel truth. >--- >Neil Desai <[EMAIL PROTECTED]> wrote: > > To my knowledge serial links don't have a MAC > > address. Since most of them > > are either a point-to-point or point-to-multipoint > > there are some other type > > of mappings. If a serial port needs a MAC address it > > usuall uses one from > > another interface that has one (i.e. ethernet). > > Neil > > ""Martinez, Carlos"" <[EMAIL PROTECTED]> > > wrote in message > > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > hello all, > > > > > > I had somebody ask me what the source mac address > > would be on a frame sent > > > across a serial link connected by to two routers, > > > for example: Host A sends a packet to Host B, > > which is on the other side > > of > > > the wan link. what would Host B see and what where > > would he send his reply > > > to.(the local router or Host A or what) > > > > > > thanks in advance > > > > > > _ > > > 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] > > >__ >Do You Yahoo!? >Yahoo! Shopping - Thousands of Stores. Millions of Products. >http://shopping.yahoo.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]
Re: Ethernet Frame (revisited for clarification)
THe LAN data link protocols have source and destination addresses. WAN protocols usually have a destination address field only (see below). >If you look at the frame format for any LAN protocol you will see where the >Destination and Source MAC address are. >If you look at >http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introwan.htm you >will see that where the WAN technologies lay in the OSI model and hopefully >this will also explain why serial lines don't have MAC addresses. >The reason that LAN protocols have a MAC address and WAN's don't is because >LAN's are contention based where WAN's are always full-duplex. Even though >layer 2 switching has been around for LAN's for a few years now the >protocols have stayed the same for backwards compatibility. >Neil > I'd disagree that WAN technologies are necessarily full-duplex. Polled, half-duplex operation was extremely common in SNA, as a means of sharing expensive dedicated lines (before frame relay and the like). Both SDLC (and its predecessors such as BSC) and LLC2 are deterministic/token-based rather than collision/contention protocols. The key difference between polled SNA and token ring, however, is control of the token. In SDLC, the token is centrally controlled (by the PU4 or PU5). In TR, control of the token is distributed. When control is centralized, and all traffic flows through the hub/mainframe, there's no need for a source address. The source address is always clear from context. There is a need for a destination address so a destination can know a poll is intended for it. So there is a need for destination addresses in WAN protocols intended for use in a point-to-multipoint environment. PPP, operating in point-to-point mode, never really needed any address field, but was designed with one because not to have one would have been incompatible with commercial data link chips of the time. Indeed, protocols such as SRP are being proposed for efficient POS applications, and these protocols have no address field because they don't need one. PS -- one thing that might be confusing about router serial lines having MAC addresses is that IPX and XNS will "borrow" a MAC address from a LAN interface in order to create the host part of a layer 3 address. _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Ethernet Frame (revisited for clarification)
Thank you for the information. Neil ""Howard C. Berkowitz"" <[EMAIL PROTECTED]> wrote in message news:p0500190eb64481a0727c@[63.216.127.98]... > THe LAN data link protocols have source and destination addresses. > WAN protocols usually have a destination address field only (see > below). > > >If you look at the frame format for any LAN protocol you will see where the > >Destination and Source MAC address are. > >If you look at > >http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introwan.htm you > >will see that where the WAN technologies lay in the OSI model and hopefully > >this will also explain why serial lines don't have MAC addresses. > >The reason that LAN protocols have a MAC address and WAN's don't is because > >LAN's are contention based where WAN's are always full-duplex. Even though > >layer 2 switching has been around for LAN's for a few years now the > >protocols have stayed the same for backwards compatibility. > >Neil > > > > > I'd disagree that WAN technologies are necessarily full-duplex. > Polled, half-duplex operation was extremely common in SNA, as a means > of sharing expensive dedicated lines (before frame relay and the > like). > > Both SDLC (and its predecessors such as BSC) and LLC2 are > deterministic/token-based rather than collision/contention protocols. > The key difference between polled SNA and token ring, however, is > control of the token. In SDLC, the token is centrally controlled (by > the PU4 or PU5). In TR, control of the token is distributed. > > When control is centralized, and all traffic flows through the > hub/mainframe, there's no need for a source address. The source > address is always clear from context. There is a need for a > destination address so a destination can know a poll is intended for > it. > > So there is a need for destination addresses in WAN protocols > intended for use in a point-to-multipoint environment. PPP, > operating in point-to-point mode, never really needed any address > field, but was designed with one because not to have one would have > been incompatible with commercial data link chips of the time. > Indeed, protocols such as SRP are being proposed for efficient POS > applications, and these protocols have no address field because they > don't need one. > > PS -- one thing that might be confusing about router serial lines > having MAC addresses is that IPX and XNS will "borrow" a MAC address > from a LAN interface in order to create the host part of a layer 3 > address. > > > _ > 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]
