Unicast, Broadcast & Multicast

[chandra_rc16]

Hi All,

I wanna learn a bit more about layer2 Broadcast & Multicast frames that a switch might receive. And how the switch process those frames.

Is there any link or doc which can explain more about layer2 unicast, broadcast & multicast.

Thanks a lot in advance.

Regards,
Chandu       

[Peter Paluch]

Hi Chandu,

I am not aware of any specific document but are you interested in anything special? A basic switch receives a multicast/broadcast frame, and when it tries to look up the destination MAC address in the CAM, it finds out it is not there (because a multicast/broadcast address is never used as a source so a switch can never truly learn it), so it floods the frame out all remaining ports in the same VLAN. It is as simple as that.

If IGMP Snooping is used, the switch snoops the IGMP Membership Report/Leave Group messages, learns about individual stations subscribing or leaving a multicast group, and derives the corresponding multicast MAC address from the contents of IGMP messages. It then adds or removes the multicast MAC address to/from the list of MAC addresses learned on the corresponding port. If a multicast frame comes in, it is then switched out only towards those stations that have truly subscribed for such multicast traffic. However, this IGMP Snooping is an added feature and not all switches support it (although all recent Catalyst switches do).

Please feel welcome to ask further!

Best regards,

Peter

[chandra_rc16]

Hi Peter,

I was taught in a institution that the while flooding switch doesn't send the frame to the source (who generated the frame).

So will a broadcast sends the frame to all the ports including the source port ? And what about the multicast too ?

Thanks in advance.

And if possible can someone please let me know the different types of MAC addresses?

Regards,
Chandu

[Peter Paluch · Accepted solution]

Hi Chandu,

I was taught in a institution that the while flooding switch doesn't send the frame to the source (who generated the frame).

That is correct. A switch never sends back a frame through the incoming port, no matter what type (unicast, multicast, broadcast) the frame is.

So will a broadcast sends the frame to all the ports including the source port ? And what about the multicast too ?

No, never - and I have never suggested that. That is why I wrote that a multicast/broadcast frame will be flooded all remaining ports, i.e. all ports in the same VLAN except the ingress port.

And if possible can someone please let me know the different types of MAC addresses?

There are not many types of MAC addresses.

In general, all MAC addresses whose first octet is an odd number is a group address, i.e. all these are group MAC addresses (x can be any arbitrary hexadecimal digit from 0 to F):

x1:xx:xx:xx:xx:xx

x3:xx:xx:xx:xx:xx

x5:xx:xx:xx:xx:xx

x7:xx:xx:xx:xx:xx

x9:xx:xx:xx:xx:xx

xB:xx:xx:xx:xx:xx

xD:xx:xx:xx:xx:xx

xF:xx:xx:xx:xx:xx

Conversely, all MAC addresses whose first octet is an even number is an individual address:

x0:xx:xx:xx:xx:xx

x2:xx:xx:xx:xx:xx

x4:xx:xx:xx:xx:xx

x6:xx:xx:xx:xx:xx

x8:xx:xx:xx:xx:xx

xA:xx:xx:xx:xx:xx

xC:xx:xx:xx:xx:xx

xE:xx:xx:xx:xx:xx

In addition, if the second lowest bit in the first octet is set, the MAC address is a local address, i.e. an address not officially assigned by IEEE. These are local individual MAC addresses:

x2:xx:xx:xx:xx:xx

x6:xx:xx:xx:xx:xx

x8:xx:xx:xx:xx:xx

xA:xx:xx:xx:xx:xx

xE:xx:xx:xx:xx:xx

There are many group MAC addresses currently registered. The most often encountered are:

01:00:5E:<0-7>x:xx:xx - IPv4 multicast

33:33:xx:xx:xx:xx - IPv6 multicast

01:80:c2:00:00:0x - IEEE 802 applications such as STP or similar

01:00:0c:cc:cc:cc - used by Cisco for CDP/VTP/DTP/PAgP/UDLD

01:00:0c:cc:cc:cd - used by Cisco for PVST+ / RPVST+

You will certainly be interested in these two documents:

https://standards.ieee.org/develop/regauth/grpmac/public.html

https://standards.ieee.org/develop/regauth/tut/macgrp.pdf

Best regards,

Peter

[chandra_rc16]

Thanks Peter.

Your explanation and the links helped me a lot.

At last one doubt remains in my mind, where will this knowledge of MAC addresses useful for me ? Whether in CCNA exam or CCNP exam or in the real life ? Have you ever got need for this indepth knowledge about MAC addresses.

Thanks in advance.

Regards,
Chandu

[Peter Paluch · Accepted solution]

Hi Chandu,

Well, I responded in depth because you asked in-depth questions But you will be perfectly safe with simply knowing that all MAC addresses carrying an odd address in their first octet are group addresses. If you happen to work more often with specific multicast applications, you may need to know them in more detail, but otherwise, do not worry too much about them.

Best regards,

Peter

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

In real-life, you don't often concern yourself with the meaning of MACs.  However, besides the grouping of MACs, e.g. unicast vs. multicast, a couple of interesting tidbits: the first part of a unicast MAC [edit: if not a locally assigned MAC] indicates the manufacturer, as blocks of MACs are assigned to insure they are unique.  Multiple blocks of multicast IP map into the same Ethernet MAC, so ideally when you use IP multicast, you want to insure your multicast IP avoids this.