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  1. As written in the title my first question is, what's the difference between Layer 2 and Layer 3 broadcasts?

  2. If a host sends a layer 2 broadcast, the switch sends it to all of his ports, but not to other switches that are connected (uplinks), that's correct isn't it? But why?

  3. How does it recognise, that there is a switch connected?

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1) Layer 2 broadcasts are sent to the broadcast mac address ffff:ffff:ffff if ethernet and so can be received by any device. Layer 3 broadcasts are sent to the broadcast network address, which for an ip network of 10.1.1.0/24 would be 10.1.1.255. If the IP network was over ethernet, a layer 3 broadcast would also result in a layer 2 broadcast.

2) A switch sends a layer 2 broadcast to all the ports that are in the same broadcast domain, ie the same vlan (which can be all ports). Some of those ports may have other switches connected, which will receive the broadcast on a port, and issue it out to all other ports on the same broadcast domain as the incoming port. So no, that isn't correct.

3) It doesn't - see 2.

Note that this answer does not discuss trunking, which adds a bit of complexity to the answer but doesn't really alter the premise.

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"A switch sends a layer 2 broadcast to all the ports that are in the same broadcast domain, ie the same vlan (which can be all ports)." What does same broadcast domain mean on Layer 2? On Layer 3 I know it's the subnet, but on Layer 2? – Ian Oct 7 '11 at 7:59
If the switch is not split into vlans, it means every device plugged into it will receive the packet. A vlan is a way of splitting a network into smaller bits, so any port in vlan1 that broadcasts will not be seen by any port in vlan2. But more than this, a switch ensures that a packet issued in vlan1 will not go to vlan2 without the help of a router (a router being a device that will pass packets between different layer 2 networks) – Paul Oct 7 '11 at 8:03
thanks, I think I understood it now. And on ethernet Layer 2 and Layer 3 broadcasts are basically the same?! – Ian Oct 7 '11 at 8:07
They are different from the perspective of the recipient. Lets say you had a host on ip 10.1.1.1/24 and another on 10.10.1.1/24, on the same broadcast domain - they are plugged into the same switch on the same vlan even though they are on different subnets. The first host issues a L3 broadcast to 10.1.1.255, which is issued to mac ffff:ffff:ffff. As the mac is a broadcast, host2 will accept the packet at layer 2. It will then go up a layer to L3, and the IP address will not be part of its L3 network (10.1.1.255 does not fall into the 10.10.1.0/24 network) and the packet will be discarded. – Paul Oct 7 '11 at 11:31
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