L3 multicast to L2 multicast conversion

ccnpninja · ‎08-11-2008

Hi,

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?

Giuseppe Larosa · ‎08-11-2008

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:

224.0.0.5

225.0.0.5

224.128.0.5 all these are mapped to:

0100.5e00.0005

Hope to help

Giuseppe

ccnpninja · ‎08-13-2008

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 · ‎08-13-2008

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

Giuseppe