Question: I am an Ubuntu Server user. I have two ethernet cards which I want to use as eth0(WAN) and eth1(LAN).
However, I have three gateways, each with a different subnet. Like this:

1. 192.168.20.12
2. 52.100.50.10
3. 10.10.1.3

I want to marge these three IPs on eth0(WAN) and how to set my eth1(LAN) gateway? 
I want this for my office because of if one or two gateways are down another is on so I never disconnect from internet.
im not understanding bridge-utils. i want details example about this that how its work?
 Please help!!

Answer: 
I'm trying to figure bridge-utils myself, and came across your post. Dear people from the future, here is what I have figured out so far:

To bridge multiple interfaces I believe you just do something like:

```
brctl addbr br0
brctl addif eth0
brctl addif eth1
brctl addif eth2

```

then you configure the address of br0

Now as to the original question here:

* from what you are asking I believe it is not really bridging. I think you just want to set multiple default gateways? This is more to do with routing rather than bridging.

Some bridging links:

<https://help.ubuntu.com/11.10/serverguide/C/network-configuration.html#bridging>
http://manpages.ubuntu.com/manpages/oneiric/man5/bridge-utils-interfaces.5.html
use `apt-get install bridge-utils` or read the info on the how to set it up ubuntu server 11.04 pages