[Bridge] mind-boggling questions

[Ireneusz Szcześniak]

Thanks, Sasikanth, for your response.  Yes, I didn't realize that we 
need a MAC address for STP.  Not only we need a MAC for a bridge, but 
there can be MAC addresses for ports too, and why I just don't know. 
I need to do some reading on STP, because I know very little about it.

On 25.04.2011 13:02, Sasikanth V wrote:
> 2011/4/25 Ireneusz Szcześniak <irek.szczesniak at gmail.com
> <mailto:irek.szczesniak at gmail.com>>
>
>     Hi,
>
>     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
>
>
>     Thanks,
>     Irek
>
>     --
>     Ireneusz (Irek) Szczesniak
>     http://www.irkos.org
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-- 
Ireneusz (Irek) Szczesniak
http://www.irkos.org