L2 Broadcasts v L3 Broadcasts

[Fuzz]

Originally Posted by bobdempsey

Considering a single-switch environment with several hosts attached, an L2 broadcast will go to all active ports. An L3 broadcast will go to all the hosts in that subnet. If all the hosts on the switch are in the same subnet then the broadcast domains are the same. If there are multiple subnets (or VLANS) then the L3 broadcast would be to a smaller set of ports than an L2 broadcast.

The L2 broadcast would not cross VLANs, so a L3 broadcast would go to the same number of ports. The difference between L2 and L3 broadcasts in in the addressing.

A unicast IP packet can be broadcast at L2 - this is how switches learn mac-port bindings. A L3 broadcast will have the destination mac address set to ff:ff:ff:ff:ff:ff so that switches treat it as a L2 broadcast to all devices on that subnet. You cannot send a L2 broadcast on VLAN 1 and hope to receive it on VLAN 2. VLANs break up broadcast domains in the same way routers do, but they do it at layer 2.

If you have multiple subnets on the same VLAN, a L3 broadcast will still be received by all devices, as it is sent by the switch as a L2 broadcast. Those devices that are not on the same subnet as the broadcast will discard the packet, but they still read it's header information.