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New Member

broadcasts

I have heard someone before talking about layer 2 broadcast, am i right in saying there is no such thing ?

6 REPLIES

Re: broadcasts

Yes there is: any frame with destination MAC ffff.ffff.ffff.

New Member

Re: broadcasts

is this called a layer 2 or 3 broadcast then, would this be a normal lan broadcast? i thouht its destined for the subnet 255.255.255.255, and not the mac address,

please explain

Silver

Re: broadcasts

Hi Carl

No, you are incorrect to say that there is no such thing as layer 2 broadcasts.

Layer 2 switches can get hung up by broadcast storms. This is something that spanning tree is designed to prevent.

See this article from Scott Morris (Quadruple CCIE & forum member) http://tcpmag.com/qanda/article.asp?EditorialsID=347

HTH

Best Regards,

Michael

New Member

Re: broadcasts

so what kind of traffic is normal on my network, layer 2 or 3 broadcast? if its a layer 3 broadcast, does it change the destination mac also? and if we do have layer 2 broadcasts, are these still broken up by vlans

?

Silver

Re: broadcasts

Hi Carl

As your network will be operating at both layer 2 and layer 3 (all your hosts have IP addresses) then layer 2 and layer 3 broadcasts will be seen and are normal.

A good example of a layer 2 broadcast is an ARP request.

A subnet level (layer 3) broadcast will be sent to the broadcast IP address of the subnet.

e.g. On the 172.16.1.0/24 subnet the broadcast IP address is 172.16.1.255

An all host (layer 3) boradcast will be sent to the destination IP address 255.255.255.255. This is for example the destination address that a DHCP discover message is sent to.

Yes, when you create VLAN's you create individual broadcast domains. Each VLAN is a broadcast domain.

Scotts article gives a really good explanation.

Best Regards,

Michael

New Member

Re: broadcasts

when you say all host broadcast, what is this, is this not the same as a subnet broadcast, if I have 2 networks say 192.168.1.0 and 192.168.2.0 if i sent a broadcast to 255.255.255.255, would they both receive it ?

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