L3 multicast to L2 multicast conversion

Unanswered Question
Aug 11th, 2008


In Cisco BSCI Official Exam Certification Guide, in "Do I Know This Already" section, page 461, I didn't understand how to answer to question 7.

Any ideas?

I have this problem too.
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Giuseppe Larosa Mon, 08/11/2008 - 10:59

Hello Wassim,

IPv4 multicast addresses are mapped to a subset of L2 MAC multicast addresses.

0x 01 00 5E + last significant 23 bits of multicast ip addresses

this means that the mapping is not one-to-one :

example: all these are mapped to:


Hope to help


ccnpninja Wed, 08/13/2008 - 00:33

thanks, but I found the answer elsewhere.

Giuseppe, you forgot to mention that the 25th bit that comes after the 0x01005E is set to 0.

Giuseppe Larosa Wed, 08/13/2008 - 04:04

Hello Wassim,

you are right it was in my mind but I didn't write it.

In fact I wrote that the last 23 bits are reported in a 24 bit wide field.

Best Regards



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