Question about switches and broadcast/multicast.

[Bob]

I believe a switch works as follows:

Each computer/device connected to the switch has it's own 'port' /
connection with the switch.

This typically gives each device connected to the switch high bandwidth.
Since only packets ment for the device are forwarded to the device by the
switch.

However how does broadcast/multicast work on a switch ?

Can a switch send the broadcast packet to all devices at exactly the same
time ?

( In this case it would only have to send 1 packet like ethernet )

Or does the switch have to send multiple packets for each device connected ?

( In this case it would have to send multiple packets one for each device
connected )

Bye,
Skybuck.

[AG]

"Skybuck Flying" <nospam@hotmail.com> wrote in message
news:cf0epr$mf6$1@news1.tilbu1.nb.home.nl... I believe a switch works as follows: Each computer/device connected to the switch has it's own 'port' / connection with the switch. This typically gives each device connected to the switch high bandwidth. Since only packets ment for the device are forwarded to the device by the switch. However how does broadcast/multicast work on a switch ? Can a switch send the broadcast packet to all devices at exactly the same time ? ( In this case it would only have to send 1 packet like ethernet ) Or does the switch have to send multiple packets for each device connected
? ( In this case it would have to send multiple packets one for each device connected ) Bye, Skybuck.

http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213079,00.html

This is a good site for all types of tech questions.
AG

[Jeff Liebermann]

On Fri, 6 Aug 2004 19:29:18 +0200, "Skybuck Flying"
<nospam@hotmail.com> wrote:
I believe a switch works as follows:Each computer/device connected to the switch has it's own 'port' /connection with the switch.This typically gives each device connected to the switch high bandwidth.Since only packets ment for the device are forwarded to the device by theswitch.

Close. A switch is a bridge with 3 or more ports. A bridge makes the
decision as to whether a packet should go accross the bridge based
upon the destination MAC address in the 802.3 ethernet header. In the
switch is a table of port numbers and which MAC addresses are found on
that port.

There are two types of bridges/switches. Store and forward is one.
Crossbar is another.
However how does broadcast/multicast work on a switch ?

Very well. A broadcast packet does not have a destination MAC
address. Therefore ALL broadcast packets are sent to ALL ports on a
switch.
Can a switch send the broadcast packet to all devices at exactly the sametime ?

In theory, yes. In reality, no. Many switches have buffer memory
(FIFO) on each port (as well as for the store and forward packet
buffers). The ability to send absolutely simultaneous packets is
therefore dependent upon what's happening in the buffer. If there's
lots of traffic, the broadcast packet will need to wait until the FIFO
buffer empties. If there's no traffic on the port, the broadcast
packet is sent immediately.
( In this case it would only have to send 1 packet like ethernet )

Not "like ethernet". It *IS* ethernet.
Or does the switch have to send multiple packets for each device connected ?

Nope, just one originating packet. Think of it as a one to many
repeater for broadcasts.
( In this case it would have to send multiple packets one for each deviceconnected )

Well, now you're getting philosophical. Once packets are repeated to
other ports, they are by definition duplicated and therefore are
classified as "multiple" packets. I think what you're asking whether
an originating ethernet device, plugged into a switch, needs to send
individual broadcasts to each connected device. No. Anything without
a desitination MAC address goes to ALL the ports. Only one
originating packet is necessary. There are exceptions as in VLAN,
Spanning Tree protocol, and DHCP relay, but we won't go there now.

--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558