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
