active dialup connection influences NIC connectivity

[Stephan Steiner]

Hi

I've discovered a weird scenario, for which I have been unable to find an
external explanation, so I think Windows is to blame and I'm wondering how
to get around it.

Here's the situation: I have computers in two subnets A and B, both part of
the class A private network addresses. The computers in subnet B have
modems connected to them, and can dial outside to customers using a standard
PPP RAS access. As soon as one of those computers in subnet B is connected
to a customer (or it could be to a normal dialup ISP), the machine is no
longer visible from subnet A. Not even a ping gets through, let alone a
remote desktop connection. As soon as the modem is hung up, the machine in
subnet A is once again accessible from subnet B. While the connection is up,
the machine in subnet B can still be pinged and accessed via remote desktop
from any other machine in subnet B.

I've set up a packet sniffer on the machine that has an active modem
connection, and found that incoming packets from machines in subnet A are
still received, but no answer is ever sent. I suspected a firewall, so I've
made sure to deactivate the firewall globally (for SP2 boxes), as well as in
the advanced properties of the dialup connection (where the firewall is
still bound to the network card and all defined dialup connections). I've
also tried the same scenario with a WinXP SP1 machine, where I once again
ensured that the windows firewall was not active. But nothing would change..
any machine in subnet B simply disappears from subnet A's reach while the
dialup connection is up.

I'm wondering, what is causing this, and how do I get around that
limitation?

Regards
Stephan

 

[Stephan Steiner]

Uh, scratch my previous message, I finally figured out what's going wrong.

Once the dialup connection goes up, I have a secondary default route with a
lower metric.. thus the reply to my pings and RDP connection attempts are
sent to the modem instead of back to where they came from. Adding a route
from subnet B to subnet A to the concerned machines in subnet B solves that
problem.

Stephan

 

[Phillip Windell]

Once the dialup connection goes up, I have a secondary default route with a
lower metric.. thus the reply to my pings and RDP connection attempts are
sent to the modem instead of back to where they came from.

Yes,...by design, on purpose, in fact.

Adding a route from subnet B to subnet A to the concerned machines in subnet B
solves that problem.

No, that is the wrong way to deal with it. Remove the route you added. All
you are supposed to do is disable the "Use Gateway of Remote Network" in the
properties of the Dial-up Connectiod. The default is for it to be enabled.

 

[Hans-Georg Michna]

Yes,...by design, on purpose, in fact.

No, that is the wrong way to deal with it. Remove the route you added. All
you are supposed to do is disable the "Use Gateway of Remote Network" in the
properties of the Dial-up Connectiod. The default is for it to be enabled.

Very good advice.

Let me just add that it may be a good idea to check whether the
subnetting has been done properly.

I personally wouldn't use a Class A network and create subnets
in it, I would prefer to use two separate Class C subnets, which
is a tad less confusing and doesn't require to create subnet
classes in address ranges that aren't meant to have them.

No big deal, if the subnetting is done properly, it should work
just as well in any address range.

Hans-Georg

 

[Phillip Windell]

enabled.

Very good advice.

I have moments,...I just gotta figure out how to make 'em last longer...

I personally wouldn't use a Class A network and create subnets
in it, I would prefer to use two separate Class C subnets, which
is a tad less confusing and doesn't require to create subnet
classes in address ranges that aren't meant to have them.

Yes, the same here. Although "classes" don't really mean anything any more,
I still like things "neat & tidy".