[Bridge] mind-boggling questions

[Joel Wiramu Pauling]

It is normal for switch vendors to assign mac addresses to a bridge -
only cheap consumer bridges don't have them, normally this is assigned
by a stanza during config.

I think many of these problems come from the fact that bridge devices
in linux are really more like a SAP construct. It would be more useful
IMHO to introduce a new class of "bridge" device which acted like a
SAP (service access port)

Consider:


Customer A - CVLAN 100
Customer B - CVLAN 200

Service A - SVLAN 1000
Service B - SVLAN 2000

Incoming interface received QinQ (stacked) 1000:[100-200] as both
subscribers are on service 1000.

To get both of these customers to be in the same segment you first
need to create a bridge, create a vlan 1000, then each CVLAN per
customer off this new interface, and finally add it to the bridge.

Tunable  like per bridge member interface broadcast enable/disable etc
would be nice as well.

A SAP construct would allow alot more flexibility and I think clear up
some of the uses that bridge devices are currently used for.



2011/4/26 Ireneusz Szcześniak <irek.szczesniak at gmail.com>:
> 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
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