swicth Broadcast

[The_guroo_2]

guys i have been listining to this broadcast concept since i have joined networking and done my CCNA,,,,,,i am bit confuse

if we have multiple vlans so first time pc will do arp so once the cam/mac table is populated then why pc will do broadcast which is according to boks sent to all port in that vlan except the one its recieved from....if mac is done then what is broadcasted all the time as in book sit says pc's/servers broadcast all the time???

Thanks

[Amit Aneja]

Let's make it simple:

PC A   -----Switch----- PC B

If PC A wants to send some traffic to PC B & it has no clue for the MAC address of PC B, then it will ARP & question everybody in the vlan that who has this IP address (PC B). this is a broadcast. Switch learns the MAC of A, updates its CAM.

PC B replies with its MAC address & so switch learns B's mac.

Now, after this broadcast thing is over, PC A can send traffic to PC B because switch know both of them.

If you flush the ARP table of switch, it has to do the broadcast thing again to learn the destination MAC.

[Leo Laohoo]

I always like a good story.  So here's my story about broadcasting in a LAN ...

Think about this:  You are in a black-tie function gala.  The waiters are dressed in duck-white and the gentlemen are dressed in tuxedo and the ladies are dressed in dresses fit for a royal wedding.  As the people mingle about (clients connected to the switch and the switch is powered on for the first time), the head waiter appears at the main door and announces "I have a message for the Duke of Archibald.  Can you please identify yourself?"  Now that is what I call as a broadcast.

The Duke of Archibald raises his hand and says "I'm here."  The head waiter goes to him, gives him the message and leaves.  A short while later, the same head waiter appears once more and announces "I have a message for the Dutchess of Cornflakes and the Duke of Archibald.  Can the Dutchess please identify yourself?".  The Dutchess of Cornflakes raises her gloved hand and says "I am here.".  The head waiter then goes to the Dutches, gives her the message and goes to the Duke of Archibald to deliver another message before leaving.

Does this small tale make any sense?

[Joseph W. Doherty]

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Posting

Regarding your mention of a switch's CAM table, the switch retains source MACs, not destination MACs.

Broadcast MACs are flooded to all ports, but ingress port, because by their nature, it's something all host's should see.

(NB: the nature of broadcasts is often the primary reason subnets can't be too large.  I.e. why "flat" network don't scale.  Broadcasts, unlike multicast and unicast MACs require hosts to process them.  I.e. they not only consume bandwidth on a switch, much like a hub, but cause host processing.  This is also why "broadcast storms" can be so detrimental to network performance.)