Overview

Mapping Multicast Addresses from L3 to L2 occurs when a layer 3 multicast packet reaches the destination subnet.

Upon reaching the destination subnet the packet no longer requires the layer 3 addressing and the packet is de-capsulated. Then, the layer 2 frame is forwarded to the corresponding multicast MAC address.

This is done for both Well-Known and Temporary multicast addresses.

In this lesson, we learn how to map the IPv6 layer 3 multicast address to the corresponding layer 2 multicast MAC address.

How it Works

Let’s use this Temporary layer 3 multicast address as an example:
FF2C::789A:BCDE

The first 16 bits of any layer 2 multicast Ethernet address will always begin with 3333.
3333

The last 32 bits of the layer 3 multicast address is appended to 3333 to form the multicast MAC address.
3333:789A:BCDE

Conclusion

That was easy!

RFC 6085: Address Mapping of IPv6 Multicast Packets on Ethernet

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