Re: [gentoo-user] Bridge confusion

[Steve [Gentoo]]

Patrick Marquetecken wrote:
> I hope that someone on this list can help me to clarify bridging.
>
I'll give that a go... someone is bound to correct me if I give you a 
partial truth. :-)

I'll start by saying that the point of a bridge (in the context of 
Ethernet networks at least) is to allow packets of any protocol type to 
be copied between two otherwise separate networks.  A bridge is a 
low-level software strategy to copy all packets from one broadcast 
network to another.  When working with IP, it is important to have a 
consistent addressing scheme - which, from your diagram, it seems you 
haven't.  I'll try a bit of ASCII art too to describe a sane situation 
in which GW_PC might be required to implement a bridge.:

     LAN1(10.32.1.0/8)           LAN2 (10.32.2.0/8)
   
     |
     +--PC1A(10.32.1.4)
     |
     +--PC1B(10.32.1.3)         PC2A(10.32.2.1)--+
     |                                           |
     +--PC1C(10.32.1.2)         PC2B(10.32.2.1)--+
     |                                           |
     +-----(10.32.1.1)---GW_PC---(10.32.2.1)-----+


Here I've illustrated 2 subnets - LAN1 : 10.32.1.0/8 (i.e. 256 addresses 
starting 10.32.1) and LAN2 : 10.32.2.0/8 (i.e. 256 addresses starting 
10.32.2.)   LAN1 has PC1A, PC1B, PC1C and GW_PC as connected hosts.  
LAN2 has PC2A, PC2B and GW_PC as connected hosts.  GW_PC might be a 
Gentoo box (or pretty much any computer you like, or a single-purpose 
device.  If GW_PC is a computer it will have two network cards - (for 
example, eth0 and eth1) and the GW_PC retransmits all packets from LAN1 
ont LAN2 and vice-versa - hence allowing the two otherwise separate 
subnets to behave as if they were a single subnet.  I use this strategy 
to bridge between my wired and wireless networks at home with my Gentoo 
PC participating in both wired and wireless networks.

It is important to realise that in copying LAN1's packets to LAN2 (and 
vice versa) that this will increase network load for both subnets - so 
strategic care is required before deciding to bridge.  An alternative is 
to route packets between the two subnets LAN1 and LAN2 - GW_PC would 
inspect the packets and decide if the destination PC is on the other LAN 
before transmitting the packet again and consuming bandwidth on both 
LANs.  Routing works well for many things (for example TCP based 
protocols) but not for all broadcast services which (without a bridge) 
are often restricted to a single subnet.

I hope that helps?

Steve

> This is the setup i want:
>
> Lan 1                                             Lan 2
>          eth1-----------brigde----------------eth1
>         |                                    |
>  10.32.0.0/22                          10.32.0.0/22
>         |                                    |
>          eth0 (10.32.3.10)-------------------eth0 (10.32.3.11)
>
>
> So the questing is, must i add eth0 and eth1 of the same machine to the
> same bridge device to get it working, or is eth1 enough ?
>
>


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