Very Computer

I am switching from isp "a" to isp "b".  Each one has it's own
dedicated router on my DMZ. I have one server (email) with one nic
card on the DMZ.  I also have a third dedicated router on the DMZ
which goes to my private network.  Isp "b" is the one I am switching
to and want everything to go through.  Routing all internet traffic
from the private network to isp "b" was simple.

        isp "a"         Isp "b"
       -----------     -----------
       | router  |     | router  |
       -----------     -----------
            |               |              ----------
             ------------------------------| router | --> private
network
                     |                     ----------
                ---------------
                | mail server |
                ---------------      

   The problem is going to be the transition period when email is
coming in through both isp's when some servers are sending to the old
ip and some to the new ip before everything is updated on the
internet.
   Currently the internet routes all my mail through isp "a" to my
dedicated ip address on the mail server and my default route on the
server is through isp "a". This works fine.  If I change my default
route to go through isp "b" I stop getting mail.  Apparently you
cannot come in through one router and return through a different one.
It seem I can only communicate with the internet through the router
which is the default route.
   By the way router "b" also has a dedicated ip but does address
translation to the DMZ network which is probably the problem.
   What I need is a way to route traffic that come from router "a"
back to router "a" and traffic from router "b" back to router "b".
   I have read many howto's and posts but none seem to address this
issue.  I was wondering if something like the following would work:
    ip route add default via router-A-ip nexthop via router-B-ip
I found a couple of posts which refered to this.  Also where can I
find a complete list of comands for ip?

   Thanks in advance for any help or suggestions you may have.

Joe D.

Do you have a seperate interface for ISP A?  Can you still connect to
ISP A's internal routers and servers - not the router in your DMZ after
setting your default route to ISP B?

Quote:>    By the way router "b" also has a dedicated ip but does address
> translation to the DMZ network which is probably the problem.

You've got to illustrate routing details before other people can
diagnose.  For example, what interfaces are there in your mail server?
What are the subnets in these interfaces? (I don't mean the values, but
the scheme, whether they are on different nets, whether each net is
public routable, etc.)

Quote:>    What I need is a way to route traffic that come from router "a"
> back to router "a" and traffic from router "b" back to router "b".

Routers "a" and "b" must be on different nets, right?  Then the only
thing you need to accomplish this is to make sure that the interface
that connects to router "a" has a route to network "a" and that
connecting to router "b" has a route to network "b".  There is no need
to even set up a default route for this specific purpose.

You really need to describe some details of your networks. (If you feel
that some information is confidential, you can either masquerade or make
clear that this is missing.  We can try asking other relavant questions.)

Yuan Liu

Quote:>    I have read many howto's and posts but none seem to address this
> issue.  I was wondering if something like the following would work:
>     ip route add default via router-A-ip nexthop via router-B-ip
> I found a couple of posts which refered to this.  Also where can I
> find a complete list of comands for ip?

>    Thanks in advance for any help or suggestions you may have.

> Joe D.