Solved: understanding of unicast, multicast and broadcast for switches

vishalpatil86 · ‎07-07-2011

hi,

In ICND 2, switching chapter, it is mentioned that "a switch or bridge never learns a broadxast or multicast address becos broadcast and multicast addresses never appear as the source address of a frame."

I dont understand this.

jputt · ‎07-11-2011

Arp broadcasts such as Arp requests are sent from a single address, the machine making the request. The source address is the machine doing the requesting, the destination is a broadcast address.

Source - Destination - Arp Request

0:80:c8:f8:4a:51 ff:ff:ff:ff:ff:ff 42: arp who-has 192.168.99.254 tell 192.168.99.35

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Peter Paluch · ‎07-11-2011

Hello,

Broadcast and multicast addresses always describe a group of recipients - more than a single receiving station. However, the sender of any frame is always a single station. A group of stations can never produce a single frame. Therefore, only the destination MAC address in a frame can be set to a broadcast/multicast address. The source MAC must always be a unicast MAC address of the network card that sent the frame. It would be totally illogical to see a broadcast/multicast MAC address written in the source MAC field.

And this is what the ICND statement tells you about. The switch always learns the MAC addresses from the source MAC field in frames. During learning, it makes a sanity check - it verifies whether the source MAC adress is a unicast MAC. If it is a broadcast/multicast, it refuses to learn it because in correct Ethernet operation, it is impossible to have a frame sent from a multicast/broadcast address.

Does this help a bit?

Best regards,

Peter

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majed.balsharaf · ‎07-07-2011

Hi Vishal

yes its true switches never learn broadcast or multicast address. Switch Learn only MAC addresses,it learn mac addresses.

frames work on mac address not on IP's. So broadcast and multicast is done for IP's and Layer 3 consists of IP's.

at layer 2 we will see mac address.

vishalpatil86 · ‎07-07-2011

So broadcast and multicast is done for IP's and Layer 3 consists of IP's.

at layer 2 we will see mac address.

what about arp broadcast then?

its layer 2

jputt · ‎07-11-2011

Arp broadcasts such as Arp requests are sent from a single address, the machine making the request. The source address is the machine doing the requesting, the destination is a broadcast address.

Source - Destination - Arp Request

0:80:c8:f8:4a:51 ff:ff:ff:ff:ff:ff 42: arp who-has 192.168.99.254 tell 192.168.99.35

Peter Paluch · ‎07-11-2011

Hello,

Broadcast and multicast addresses always describe a group of recipients - more than a single receiving station. However, the sender of any frame is always a single station. A group of stations can never produce a single frame. Therefore, only the destination MAC address in a frame can be set to a broadcast/multicast address. The source MAC must always be a unicast MAC address of the network card that sent the frame. It would be totally illogical to see a broadcast/multicast MAC address written in the source MAC field.

And this is what the ICND statement tells you about. The switch always learns the MAC addresses from the source MAC field in frames. During learning, it makes a sanity check - it verifies whether the source MAC adress is a unicast MAC. If it is a broadcast/multicast, it refuses to learn it because in correct Ethernet operation, it is impossible to have a frame sent from a multicast/broadcast address.

Does this help a bit?

Best regards,

Peter

vishalpatil86 · ‎07-11-2011

it helped a lot.

thank u jeff and peter