On Tue, 19 Aug 2003 11:17:33 +1000 Andrew White <[EMAIL PROTECTED]> wrote:
> "Bound, Jim" wrote: > > > > Below is a picture of two links: Link 1 and Link 2. Link 1 has > > Host-L1-B and Host-L1-C. Link 2 has Host-L2-E and Host-L2-F. > > A multihomed Host-LX-D0 is connected to both Link 1 and Link 2. > > All hosts have both a Link-Local address FE80::XXXX and a Global > > Address 3FFE:YY::XXXX. Note that Host-L1-B and Host-L2-E have the > > same Link-Local address as FE80::MAC1. This is permitted in IPv6 > > for separate links. > > It was my understanding that duplicate MACs are not permitted by the MAC > allocations, which are *supposed* to guarantee that each physical interface > device has a unique MAC. > > Which would imply that a link-local addressed based on a MAC is in fact > globally unique, by definition? > > What have I missed? > a) MAC addresses are reasonably easily to change. You can't guarantee that the end-users will follow the recommendations ie. enable the locally administered bit. Most networking people I've met don't know there is a "locally administered" bit in a MAC address. b) There have been cases where manufacturers have allocated non-unique MAC addresses. What is worse is that these duplications have apparently tended to happen within the same batch of NICs, and have been encountered when somebody goes to deploy a group of say 20 new NICs they have just bought, and encounter one or more duplications. c) MAC addresses are typically placed in outgoing ethernet frame headers by the device driver, not by the NIC itself, which is why it changing the MAC address of an interface is usually functionally quite easy. The device driver gets the MAC address to use from an EEPROM or something similar from the card. Bugs in that part of the device driver can cause duplicates. You can generally _assume_ MAC addresses are globally unique, which is why they are used as node IDs in IPv6, IPX, but there aren't any guarantees. Further, global uniqueness of MAC addresses is really only so that the device can be plugged in and work "out of the box". For the network to actually work, the MAC address only has to be unique on the segment it is attached to. Regards, Mark. -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
