switching ISP's

switching ISP's

Post by Jon LaBad » Mon, 06 Jan 1997 04:00:00

Not a current problem, asking for input for a planned change.
What should I worry about?

I've been connected to the net via slip and my ISP "jvnc" for
several years.  Thus I can be contacted via email at:

But only jgcomp.jvnc.net is a valid site for other uses. i.e.
jgcomp.com is only and MX record, for telnet ... jgcomp.jvnc.net
must be used.

I have several systems on my local net AND I have a properly
registered domain jgcomp.com.  However my ISP will not route
traffic to jgcomp.com without a serious upgrade in service and
price.  Only email via the MX record.

So, I'm looking for an alternative ISP.  One who will use my
registered domain.  During setup with a new ISP I'm concerned
about name conflicts.  I hope to run both connections for a
short period.

What will happen to email (etc.) if there is an MX record
directing it to my current ISP and a domain record directing
it to my new ISP?  Will I also have to ask for mail to go to
a specific machine within my domain rather than the current
system where there is essentially only one connected machine?

BTW during the test period the two connections will be through
two different systems, jg1 to one ISP and dutch to the other.


 4455 Province Line Road        (609) 252-0159
 Princeton, NJ  08540-4322      (609) 683-7220 (fax)


1. transition from isp 'A' to isp 'B' routing problem

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
                     |                     ----------
                | 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
   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.

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