Hi Patrice, In my example, I was leaving out the OUI portion since that will always be 01-00-5E. The full L2 MAC would have been 01-00-5E-04-05-05.
David. On Fri, Jun 24, 2011 at 12:43 AM, Patrice Ngassam <[email protected]> wrote: > Hi Dave, > I am pretty sure we will have a new RFC very soon regarding Multicast MAC > address derivation from the multicast IP address. While looking at the > example you shown below, it looks like your I/G bit is 0 instead of 1. Any > thoughts about that? > > Regards, > > Patrice Ngassam > CEO NEN NET Inc. > > > >> Date: Thu, 23 Jun 2011 19:29:09 -0400 >> From: [email protected] >> To: [email protected] >> Subject: [OSL | CCIE_RS] a quick method for converting multicast L3 to L2 >> MACs >> >> Hi CCIE R&S List, >> >> I'm in the final stages of prep for the IE written and I believe I've >> figured out a way to map L3 multicast addresses to L2 multicast MAC >> addresses, for the purpose of comparing overlap, in a much quicker way >> than I've seen presented before. I'm writing to share my findings and >> to see if anyone views any flaws in this method. >> >> I've always read that the lower 23 bits are used in the conversions >> and the remaining (higher) 9 are simply discarded by their >> substitution with a single hex 0. What's interesting though, is that >> the higher 3 bits of that remainder are also discarded with no >> substitution. So in effect, we really only have a four total bits to >> evaluate, positions 16-19, because positions 0-15 would have to be >> identical to produce the same hex values on conversion. >> >> >> Here's an example: >> >> 224.132.5.5 >> Step 1) Ignore the 224, because it gets discarded. >> >> Step 2) Subtract 132 from 128, which results in 4. >> Since only the lower half of this octet will be used in conversion, we >> only care about values of 16 and below. Convert the lower half to >> hex, which results in a value of 4. >> >> Step 3) Prepend a 0 to the above hex value. The 0 replaces the >> higher 12 bits of the original address. >> >> Step 4) We now have 04 as the significant hex value of the L2 MAC. >> >> Step 5) All remaining positions are not relative to the problem at >> hand, as they must match to create an identical hex result. >> >> Step 6) The resulting significant portion of the L2 MAC is 04-05-05. >> >> >> >> 239.68.5.5 >> Step 1) Look only at the third octet, the 68. >> Step 2) Subtract 64 from 68, which results in 4. >> Step 3) Prepend 0 to the above value, resulting in hex value 04. >> Step 4) The resulting significant portion of the L2 MAC is 04-05-05. >> >> >> >> 226.36.5.5 >> Step 1) Look only at the third octet, the 36. >> Step 2) Subtract 32 from 36, which results in 4. >> Step 3) Prepend 0 to the above value, resulting in hex value 04. >> Step 4) The resulting significant portion of the L2 MAC is 04-05-05. >> >> 239.132.5.5, 239.68.5.5, and 226.35.5.5 all result in overlapping >> layer 2 multicast MAC addresses. >> >> >> David. >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, please >> visit www.ipexpert.com >> >> Are you a CCNP or CCIE and looking for a job? Check out >> www.PlatinumPlacement.com > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
