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Flooding vs Broadcast

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  • Flooding vs Broadcast

    Hi dears,Today i read that if a switch does not know destination address then it broadcast or flood to all ports except the port from where frame was received.But question is that if a switch has 24 ports and suppose it has learnt MAC addresses of (suppose first)12 ports,and if it receives frame destined for port 13 whome MAC address entry is not in MAC address table then what it will do? It will just broadcast or flood to all ports except the port from where this frame was received or the ports from 13-24 only (as it already has MAC addresses entry for first 12 ports).According to me Flooding is " broadcast to unknown ports "and Broadcast is simply "broadcast to all ports".So if i m correct then the switch should flood frames,mean it should flood to ports 13-24 and not to all including 1-12.if i m wrong then what is the difference between flooding & broadcast.Any suggestion is welcomed.
    Last edited by zahid3963; 10th December 2009, 16:41.

  • #2
    Re: Flooding vs Broadcast

    flood/broadcast mean the same thing.

    A switch will flood to all ports in a VLAN if the destination is unknown. It does not matter if theres a MAC to port relationshio already, a port can have more than one MAC.

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    • #3
      Re: Flooding vs Broadcast

      In the case you provide broadcast is sent to all devices in a broadcast domain. Broadcast domains can be separated by layer 3 /network layer of the OSI model/ devices such as routers or via vlan /which technically is broadcast domain/. Anyway there is layer 3 broadcast also.
      In the link below you can find more specifically the difference between udp flooded and directed broadcast.

      http://www.cisco.com/en/US/docs/inte...ies/cs006.html

      further reading ieee 802.1d MAC bridges standard:
      http://standards.ieee.org/getieee802...02.1D-2004.pdf
      Last edited by alexbg; 10th December 2009, 19:10.

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      • #4
        Re: Flooding vs Broadcast

        Originally posted by zahid3963 View Post
        According to me Flooding is " broadcast to unknown ports "
        Flooding is the process where a switch sends data destined for an unknown MAC address to all available ports on the device.



        and Broadcast is simply "broadcast to all ports".
        broadcast will send all data to all addresses within the broadcast range.
        For instance - 192.168.1.1/24 - 192.168.1.254/24 is in one broadcast range.
        however 192.168.2.1/24 - 192.168.2.254/24 is in a different broadcast range, so even if you're all plugged into the same switch, it will only send to one set of addresses.

        A broadcast domain is based on, funnnily enough, the broadcast address..

        With a HUB, all it ever does it flood/broadcast - it sends everything, back out every port, because it has no way of undesrtanding
        not many places use hubs any more though
        Please do show your appreciation to those who assist you by leaving Rep Point https://www.petri.com/forums/core/im.../icon_beer.gif

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        • #5
          Re: Flooding vs Broadcast

          To clarify: broadcasts can occur at the data link layer or the network layer. A broadcast at the data link layer (MAC address ff-ff-ff-ff-ff-ff) would be "flooded" or broadcast to every switch port regardless of what subnets the hosts were on at layer 3.

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          • #6
            Re: Flooding vs Broadcast

            joeqwerty has synthesized the explanation very good. Just want to add that in link layer broadcast the switch is flooding the broadcast /ff:ff:ff:ff:ff:ff/ to the ports in the same vlan where it comes from. If you have ports in other vlans on the same switch they will not be flooded with this broadcast because they are in different broadcast domain.

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            • #7
              Re: Flooding vs Broadcast

              Originally posted by alexbg View Post
              joeqwerty has synthesized the explanation very good. Just want to add that in link layer broadcast the switch is flooding the broadcast /ff:ff:ff:ff:ff:ff/ to the ports in the same vlan where it comes from. If you have ports in other vlans on the same switch they will not be flooded with this broadcast because they are in different broadcast domain.
              Thanks, I forgot to mention that.

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              • #8
                Re: Flooding vs Broadcast

                Thanku dear,i got answer from another forum.I actually need to know difference between broadcast and flooding.Flooding is type of broadcast in order to learn or get some thing,like switch does to learn destination MAC address when it receives frame for unknown MAC address.And Broadcast is simply broadcast to all ports but not intented to get some result or ,like, learn some MAC address.So in LAN ,switch does Flooding in order to know MAC address & not broadcast.It also does broadcast but that is not to know MAC address.

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                • #9
                  Re: Flooding vs Broadcast

                  Again, in this context the words are interchangable.

                  "Flood" is the action a switch will take on a frame with a broadcast or unknown destination address.

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