[Bridge] mind-boggling questions

[Bridge] mind-boggling questions

[Ireneusz Szcześniak]

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

2. Why does a bridge take the lowest MAC address of the interfaces 
connected to it?

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?


Thanks,
Irek

-- 
Ireneusz (Irek) Szczesniak
http://www.irkos.org

Re: [Bridge] mind-boggling questions

[Sasikanth V]

[-- Attachment #1: Type: text/plain, Size: 2635 bytes --]

2011/4/25 Ireneusz Szcześniak <irek.szczesniak@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
> _______________________________________________
> Bridge mailing list
> Bridge@lists.linux-foundation.org
> https://lists.linux-foundation.org/mailman/listinfo/bridge
>

[-- Attachment #2: Type: text/html, Size: 4005 bytes --]

Re: [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@gmail.com
> <mailto:irek.szczesniak@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
>     _______________________________________________
>     Bridge mailing list
>     Bridge@lists.linux-foundation.org
>     <mailto:Bridge@lists.linux-foundation.org>
>     https://lists.linux-foundation.org/mailman/listinfo/bridge
>


-- 
Ireneusz (Irek) Szczesniak
http://www.irkos.org

Re: [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@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@gmail.com
>> <mailto:irek.szczesniak@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
>>     _______________________________________________
>>     Bridge mailing list
>>     Bridge@lists.linux-foundation.org
>>     <mailto:Bridge@lists.linux-foundation.org>
>>     https://lists.linux-foundation.org/mailman/listinfo/bridge
>>
>
>
> --
> Ireneusz (Irek) Szczesniak
> http://www.irkos.org
> _______________________________________________
> Bridge mailing list
> Bridge@lists.linux-foundation.org
> https://lists.linux-foundation.org/mailman/listinfo/bridge
>