Re: mind-boggling questions

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2011/4/25 Ireneusz Szcześniak <irek.szczesniak@xxxxxxxxx>

I have a few questions on the Linux bridge, and I would appreciate it
if someone could answer them.

1. Why does a bridge have a MAC address?  A bridge doesn't need a MAC
address. I understand that a Linux box might offer more than a regular
switch, and for that you need a MAC address. But the services should
    Yes, Linux bridge needs mac address. Because linux works based on the 802.1D bridge
    which has MAC relay entity,  Spanning tree protocol entity. The bridge required mac-address to communicate
    with other bridges. For example to form a loop-free network spanning-tree protocol in the bridges will communicate with
    the bridges using BPDU, the bpdus carry the bridge address to identify from which bridge we have received the BPDU.
    If your bridge just acts as forwarding  agent, then mac address is not requried.

be provided by a new tap interface added to the bridge. I believe that
the bridge should not even be shown by ifconfig.

    I am not familiar with tap interface, So ignoring it.
    I agree with you that bridge should not be shown in ifconfig. Bridge must come up  when anyone of the bridge ports are up.
    But in current implementation we have to issue ifconfig <bridge> up  to make the bridge up. As far as i see it is not required.
    Hope someone can give more clarification and its purpose
2. Why does a bridge take the lowest MAC address of the interfaces
connected to it?
       From 802.1D

       7.12.5 Unique identification of a bridge
       A unique 48-bit Universally Administered MAC Address, termed the Bridge Address, shall be assigned to
       each Bridge. The Bridge Address may be the individual MAC Address of a Bridge Port, in which case, use
       of the address of the lowest numbered Bridge Port (Port 1) is recommended.

3. When I send broadcast frames to a bridge interface (etherwake -b -i
br0 00:00:00:00:00:00), the frames are received by the interfaces of
the bridge.  But when I send the frames to one of the interfaces, they
are not broadcasted to other interfaces.  I though that the bridge
interface (br0) behaves the same as an interface added to the bridge,
but I was mistaken.  What are the differences?

    no idea


Ireneusz (Irek) Szczesniak
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