I have 2 ISPs having router-IPs 172.16.1.10 & 172.16.1.253.
Server's present route is 172.16.1.10, when it goes off server should switch automatically to second one ie 172.16.1.253

Plz. guide me to add both gateways in centos

How many NIC's do you have?

If there's a single nic, I'm not sure that you can do it... Well, not without scripting up a constant icmp check to the 172.16.1.10 address, and if it fails, then have the script change a file for you.

The absolute easiest way to do it (Without setting any strange routes), I would:
cp /etc/sysconfig/network /etc/sysconfig/network.orig
vi /etc/sysconfig/network ---> Change the default gateway in here

When the gateway fails, then you can have a script to switch the two files over for you and restart the network service:
ie.

mv /etc/sysconfig/network /etc/sysconfig/network.orig1
mv /etc/sysconfig/network.orig /etc/sysconfig/network
mv /etc/sysconfig/network.orig1 /etc/sysconfig/network.orig
service network restart

The above assumes that you haven't set up your gateways in /etc/sysconfig/network-scripts/ifcfg-eth0
It also assumes you haven't set any routes for info.

Don't forget to test it before it's implemented.

Hope that helps somewhat.

Thanx for reply.

In have a single NIC, and one gateway is already defined in \etc\sysconfig\network\ifcfg-eth0 and no default gateway in \etc\sysconfig\network. still can I use your given commands

to add multiple defult gateways.

Plz. Help me.
Thanx
ssy

You may check this:

http://code.google.com/p/wanfailoverscript/

or

http://joel.amw.co.id/w/failover-scr...y-in-linux.txt

But this assumes you have two external interface connected to the ISP.

Thanks!

You can use the same commands, just change it to the ifcfg-eth0 instead of the 'networks' file.

And as I mentioned, you can do it with magical routes, as mmheera has suggested.

Good luck.