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Re: Is flooding and Broadcast same in a switch [7:89368] posted 06/08/2004
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Sachin Walia,

As Patrick pointed out, the switch does not change the destination mac
address of user traffic.  The switch floods all frames where there is no
match between the destination MAC address and the addresses in the
mac-address table.

If the mac address table is empty, it will flood everything.  By the same
token, if you statically associate the broadcast mac address to a particular
port, the switch will no longer flood broadcasts.

The switch builds its mac address table by observing source addresses in
user traffic.  Since broadcast and mulitcast (group) addresses never show up
as source addresses, it does not learn to associate them with particular
ports, and floods them.  The switch can also learn from the administrator,
via static configs, and through advanced protocols like CGMP and IGMP
snooping, so that group addresses are not flooded.

HTH,

Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net


----- Original Message ----- 
From: "sachin walia" 
To: 
Sent: Tuesday, June 08, 2004 2:06 PM
Subject: Is flooding and Broadcast same in a switch [7:89368]


> Hi,
>
> I have read that,if a switch does not know the destination (MAC address
say
> XYZ) it floods the frame out to all its ports except the one on which it
was
> received.
>
> Is this flooding same as Broadcast i.e
> (i) does it use all FFs in the destination MAC address, or
>
> (ii) uses the original destiantion MAC address (XYZ).
>
> in the first case each host on each segment will receive the frame and
only
> on basis of L3 address will an individual host determine whether the
packet
> is destined for it. (This will though require unnecessary processing by
> other hosts who cannot discard the frame just by checking the MAC
address).
>
> in the second option each host would still receive it but will only
compare
> the MAC address and discard the frame if it does not match, hence keeping
> the decision to layer 2 only.
>
> which one is the correct,
>
> Comments are appreciated
> Sachin




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