Broadcast or Flood ?

Answered Question
Nov 5th, 2007

Is there a difference between broadcasting a network and flooding a network ? I have seen both terms used in my studies.

Correct Answer by Kevin Dorrell about 9 years 3 months ago

Briefly: a broadcast is something that is addressed to everybody, and therefore gets sent to everybody.


A flood is something that is sent to everybody. It may be because it is a broadcast, that is addressed to everybody. Or it may be addressed to one specific destination address, but you do not know where that destination address is, physically.


Simplistically:


"All passengers please do not leave your baggage unattended." is a broadcast - it is also flooded.


"Mr. Smith, please contact the information desk" is flooded, but it is addressed to one person, so is not really a broadcast. Everyone hears it, but only Mr. Smith needs to react.


"Passengers for flight XYZ, please report to gate 12" is a directed broadcast, or maybe a multicast. Everyone hears it, but only the passengers concerned need to react.


Kevin Dorrell

Luxembourg


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Correct Answer
Kevin Dorrell Mon, 11/05/2007 - 06:08

Briefly: a broadcast is something that is addressed to everybody, and therefore gets sent to everybody.


A flood is something that is sent to everybody. It may be because it is a broadcast, that is addressed to everybody. Or it may be addressed to one specific destination address, but you do not know where that destination address is, physically.


Simplistically:


"All passengers please do not leave your baggage unattended." is a broadcast - it is also flooded.


"Mr. Smith, please contact the information desk" is flooded, but it is addressed to one person, so is not really a broadcast. Everyone hears it, but only Mr. Smith needs to react.


"Passengers for flight XYZ, please report to gate 12" is a directed broadcast, or maybe a multicast. Everyone hears it, but only the passengers concerned need to react.


Kevin Dorrell

Luxembourg


jshanley7 Mon, 11/05/2007 - 06:49

So if a switch receives a unicast address that is not in the mac table and sends it out all ports except the one it received, that is considered a flood not a broadcast ? Thanks in advance.

Kevin Dorrell Mon, 11/05/2007 - 06:56

That is indeed a flood. The frame is still addressed to the single unicast destination, but is sent everywhere because you don't know yet which port the destination is on.


Kevin Dorrell

Luxembourg


scottmac Mon, 11/05/2007 - 07:53

There is a big difference, with respect to a switch flooding versus a switch broadcasting.


A broadcast frame has a destination address of all ones.


With a flood, the switch is trying to find the destination host when it doesn't know where it is, so the switch "floods" the frame out all ports ... the destination address remains intact (NOT ALL ONES, like a broadcast).


Only the intended host / receiver will pay attention to the flood, because it contains the destination host's address.


With a broadcast, the frame must be passed up the stack by every host that hears it .. the host may toss it, but it first has to evaluate it.



So, the difference is:

Broadcast: destination all ones

Flood: destination address remains the same / as received.


Good Luck


Scott


cosminvs Mon, 11/05/2007 - 09:51

I assume you use the term network referring to a LAN or VLAN (which, remember, is best defined as a broadcast domain).

There's no such thing as "flooding a network" but there might be such thing as "flooding" mechanism triggered by the same frame in more than one switch or even in all switches composing a LAN (network as you said).

The term flooding, in networking people acceptation, applies to a L2 switches while the term "broadcast" applies to the LAN in its entirety which is a summum of L2 elements (switches and host's NICs). You may compare, inside a switch only, the treatment of a broadcast frame with the operation of flooding mechanism only in the particular case when the LAN is composed by a sole L2 switch.

At host's NIC level a received broadcast frame is always passed upwards while a frame received due to the "flooding" mechanism is discarded unless the respective NIC is also the destination for that frame or works in promiscvous mode.

Maybe the best way to perceive the difference is to meditate to the basic definition of VLAN as a broadcast domain.

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