When trying to bring up my 2nd interface up I noticed a strange problem. 
Let me try to explain exactly what I was doing. My real IPs have been 
changed.

Currently we have eth0 up and connected to a 
network, /etc/sysconfig/network-scripts/ifcfg-eth0 reads:

DEVICE=eth0
BOOTPROTO=static
IPADDR=1.2.3.80
NETMASK=255.255.255.192
ONBOOT=yes

and /etc/sysconfig/network reads:

NETWORKING=yes
HOSTNAME=bb0.mydomain.com
GATEWAY=1.2.3.65

Currently, there is an external machine 1.2.4.3 which I can ping and have 
access to.

Now I created /etc/sysconfig/network-scripts/ifcfg-eth1 to read:

DEVICE=eth1
BOOTPROTO=static
IPADDR=1.2.4.3
NETMASK=255.255.255.0
GATEWAY=1.2.4.1
ONBOOT=no

This interface has no cable connected to it. I then brought up the 
interface with ifup eth1, and it failed on adding the route with network 
unreachable. Now 1.2.4.3 goes to the local machine and is no longer the 
external one.

Next, I did ifdown eth1, verified that ifconfig did not show eth1 as being 
up, then tried to ping 1.2.4.3. It was then still going to the local 
machine (I verified it was local machine from the services running and 
ping times), instead of the external 1.2.4.3 as it did before.

In order to get access back to the external 1.2.4.3 machine, I changed 
eth1's IP to be 1.2.4.4, brought it up, then back down, and everything was 
ok.

This does not seem normal to me, am I doing something wrong when bringing 
the interface down? ifconfig -a did not show it as being UP when I could 
still access 1.2.4.3 as the local machine.

Any help appreciated.

Viraj.