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I am confused with the following 2 cisco statement regarding broadcast. Pls help to clarify?
12-15-2012, 10:52 AM
Post: #1
I am confused with the following 2 cisco statement regarding broadcast. Pls help to clarify?
1)A broadcast frame is never forwarded by a switch
2)The router by default will never forward broadcast

Which one is correct? or are both correct? I think a switch can forward broadcast frames but I read they don't, pls clarify

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12-15-2012, 11:00 AM
Post: #2
 
A router will never forward a broadcast because broadcasts should never leave the local net.
A switch is part of the local net, so by default it will forward everything.
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12-15-2012, 11:00 AM
Post: #3
 
Both statements are true.

I switch will never forward a broadcast frame instead it will flood the frame.

A forward occurs when the switch receives a frame and looks up the destination mac address and the mac address is connected to a port other then the one it came in on. It then forwards the frame out that port.

A filter occurs when the swich receives a frame and it looks up the destination mac address and the mac address is connected to the some port the frame was received from. It then drops the frame.

A flood will occur when the switch receives a frame and does not have a entry in its mac address table. It will then send the frame out every port except the one it came in on. Therefore the action a switch takes on a broadcast frame is flood NOT foward.

A switch will NEVER send a frame out the same port it came in on. In networking there are not very many absolutes... but in this case there is one.

A router will listen to broadcast but will never 'route' them. One of the functions of a router is to filter broadcasts thereby splitting up broadcast domains. If routers were to forward broadcasts then the internet would come to a screeching halt. Broadcasts packets or frames are meant to find information on the local network not across different networks.

Its hard for me to explain #2... just know that by default a router will not forward a broadcast.

Good Luck,
Rick

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