Re: [FW-1] Two ISPs

[Crist Clark <crist.clark@FW1-NOSPAMGLOBALSTAR.COM>]

Brad Pinkston wrote:
>
> We are currently switching ISP and must reroute the /24 network we own.  Is
> there any way to configure these lines to work together as the route change
> is taking effect with Internic?
>
> I wasn't sure if this would work:
> 1) Configure both interfaces as default routes with old ISP higher priority
> (Is this how static routes priority works on Nokia?)
> 2) As soon as the route change takes effect traffic should continue on new
> line (Or will our network traffic still go outbound on old line because of
> priority?)
>
> The only need for BGP would be if one line went down so all our subnets
> could work on both ISPs, correct?
>
> Any help on this matter would be greatly appreciated.

>From a routing point of view, there really isn't anything special you
need to do. The ISPs should be taking care of everything. (And good luck
with that all happening smoothly. Heh.) Once the link with the new ISP
is up and running, you can send all of the packets out that way. Until
it up, don't use it. As the new route to your network propagates through
the global BGP tables, the returning traffic will shift from coming in
through old ISP to new ISP. Just remember that which way the packets go
out has absolutely nothing to do with what route the responses come
back, i.e. you cannot influence the return path by chosing which link
you transmit from. There really is no point in trying to use both at once.

Now, from the firewall's perspective, this may be trouble. Since you can
have packets go out of the new link and come back in the old. (Or the other
way around if you wait to switch to the new link until after the new ISP
starts advertising your address block.) You may need to disable anti-spoofing
to get the firewall to pass that kind of traffic. It would otherwise complain
that the responses are coming into the wrong interface.

If you want more help with that, more about your topology and your FW-1
version would be necessary.
--
Crist J. Clark                               crist.clark@globalstar.com
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