Multicast IP Address to MAC address mapping

I have a router on a stick lab. 1 2960 divided into 2 vlans connected to router subinterface. Is there any commands that need o be configured on a switch for multicast traffic?

Hi Jason,

Do you want to use multicast within the VLAN or route multicast traffic between VLANs?

There’s not much you have to do on the switch, you might want to read up on IGMP snooping to see how it works:

IGMP snooping is enabled on most switches by default.

Rene

In terms of Layer 2 multicast addresses, it seems there are some exceptions to the 0100.5e.X prefix rule?

0100.0ccc.cccd is for PVST and is considered multicast
0180.c200.0000 is for regular spanning tree and considered multicast

I suppose these will never map to Layer 3 addresses so that’s probably why they appear random.

Hello Chris

You are correct that these layer two addresses are indeed multicast addresses. However, these function for exclusively layer two protocols such as PVST, STP, CDP, VTP, UDLD and others. This means that there is no corresponding multicast IP address to map them to. Multicast IP addresses are only mapped using the stated rule.

I hope this has been helpful!

Laz

That exactly is my doubt

Since some Multicast IPv4 addressses map to the same multicast mac add :slight_smile:
if we have to host H1 & H2, H1 joins to 239.1.139.1 and H2 joins to 224.1.139.1, both are on the same segment, but H1 also will receive multicast packets for the other group (224.1.139.1) and also H2 will receive L2 multicast traffic for the other group (239.1.139.1) … that’s right ?

But H1 Nic interface will drop this l2 multicast traffic ? unlike broadcast traffic that will be passed through the nic to the cpu ?

Hello Juan

Yes that is correct. But in order to have duplicate packets a lot of unlikely things have to happen simultaneously, so it is a rare occurrence. If the network administrator is on top of things, he or she will make sure to use appropriate group addresses to avoid such a situation.

Yes, the NIC will indeed drop this multicast traffic. Remember that the NIC is “multicast aware” so it knows to which multicast groups it belongs. If it receives multicast traffic with a destination multicast IP address different from those groups to which it belongs, it will drop the packet.

I hope this has been helpful!

Laz

Ask me a question please. Throughout the text we say that the MAC address has 48 bits and that the first 24 bits are fixed for multicast addresses.
There are 24 bits left. My question is why is it said that half of these remaining 24 bits are 23 bits?
I confess that I am confused by the explanation.

Hello Marcelo

Yes this is indeed confusing. Remember that we’re talking about binary. If you have 24 bits available, you can represent 2^24 numbers, which is numbers between 0 and 16,777,216. If you take away the leftmost bit from these 24 bits, you’re left with 23 bits. This means you can represent 2^23 numbers, or numbers between 0 and 8,388,608.

8,388,608 is half of 16,777,216.

Every time you reduce the number of bits representing a binary number, you divide it by two which is the base of the binary system. The same happens with decimal, but in decimal, you divide by 10 because 10 is the base of the decimal system. For example, if you have the number 9999, and you remove one digit, you get 999 which is 10 times smaller.

I hope this has been helpful!

Laz