cable modem w/ISDN back-up connection: how to ?

cable modem w/ISDN back-up connection: how to ?

Post by John Gil » Thu, 25 Jun 1998 04:00:00



Hi all.

We are installing a cable modem for Internet access on our LAN,
replacing our ISDN connection.  We have an NT tcp/ip network. We
currently use an Ascend router for ISDN.  I would like to have the LAN
connect to the cable modem, but use the ISDN router for a back-up, in
case of connection problems on cable.

Can anyone point me in the direction to set this up.  I know that I can
manually configure the ISDN router to change IP addresses on the LAN if
the cable connection fails, but I was thinking about an automated
solution that would access the LAN IP address of the NIC connected to
the cable modem first, and after a period of time, if no connection was
found, to route to the LAN IP address of the ISDN router.

Any suggestions will be welcomed.

John

 
 
 

cable modem w/ISDN back-up connection: how to ?

Post by John Gil » Fri, 26 Jun 1998 04:00:00


> Hi all.

> We are installing a cable modem for Internet access on our LAN,
> replacing our ISDN connection.  We have an NT tcp/ip network. We
> currently use an Ascend router for ISDN.  I would like to have the LAN
> connect to the cable modem, but use the ISDN router for a back-up, in
> case of connection problems on cable.

> Can anyone point me in the direction to set this up.  I know that I can
> manually configure the ISDN router to change IP addresses on the LAN if
> the cable connection fails, but I was thinking about an automated
> solution that would access the LAN IP address of the NIC connected to
> the cable modem first, and after a period of time, if no connection was
> found, to route to the LAN IP address of the ISDN router.

> Any suggestions will be welcomed.

> John



 
 
 

cable modem w/ISDN back-up connection: how to ?

Post by Sam C. Nicholson ! » Fri, 26 Jun 1998 04:00:00


John,

Unless one box can terminate both the ISDN and Cable connex,
you need a third box to make routing decisions for your lan.

NB That third box can be contained within one of the other
   two.  But for sake of discussion, its best to think of
   it as a separate entity.

Now in the traditional dedicated world, one would multi-home
via leased lines, and run BGP to manage the routes.  This
also has the advantage of sending packets to the provider
that is better able to deliver them.  And if either line
goes down, the BGP session shuts down, and those routes
are withdrawn from your routers table.

I rather think it unlikely that a cable ISP or a dial-up
ISDN ISP will offer BGP.  First off you have to have your
own AS and you have to have you own address block.

Unfortunately the shortage of available network blocks
means that many of the "appropriate" solutions to everyday
problems are not available to everyday folks.

Again, one of your routers may be smart enough to do this
on its own, but I am going to talk as if you had a third box.

What you do is have a router talking to both boxes.  Here,
a UNIX box makes sense for the router, it can be an old 486
with 2 ether cards, for it will need to run some scripts that
check on the outgoing connections, and tickle the appropriate
outbound router.  The third box will act as a router to your
LAN, and will have separate views of the two outgoing boxes.

The script will periodically test the primary line, and, if
it is down for more than some specified time, it will send
packets to the backup router.

Assuming that you can't get routing from two such providers
to an address block that you own,  the third box will need
to run either NAT or DHCP, so that the operational results
are appropriately reflected to your lan.

-sam

 
 
 

cable modem w/ISDN back-up connection: how to ?

Post by Laurence V. Mar » Mon, 29 Jun 1998 04:00:00



Quote:>Hi all.

>We are installing a cable modem for Internet access on our LAN,
>replacing our ISDN connection.  We have an NT tcp/ip network. We
>currently use an Ascend router for ISDN.  I would like to have the LAN
>connect to the cable modem, but use the ISDN router for a back-up, in
>case of connection problems on cable.

I can only speak in generalities on this, John, but it might be enough to get
you looking in the right place.  Routers have routing tables, in which you can
specify gateway routers and cost of connections.  I am assuming your local
network is 10.x.x.x or 192.168.x.x.  Assume for a second that you have:
--A standalone router (e.g., an IBM 2210) (with its own IP address)
--An ISDN router (with its own IP address)
--A cable modem (with its own IP address)

Configure all the workstations to send everything off the local (10. or
192.168.) to the standalone router.

Configure the standalone router with:
--Local is 10.x.x.x or 192.168.x.x
  (implies netmasks of 255.0.0.0 or 255.155.0.0)
 --Otherwise:   route over cable modem with Cost of 1
               route over ISDN router with Cost of 50

Now you probably cannot configure routing tables in the cable modem, but a real
interesting exercise for the student (that's you) is to see whether you can
configure routing tables in the Ascend ISDN router so that all workstations
send to it, it re-routes to the cable modem (cost 1) and out the ISDN port
(cost 50).  Kevin, are you out there?  Can John's router do this?  If it can,
you don't need a third standalone router, and the silght latency it adds.

Laurence V. Marks
IBM Corp. - Research Triangle Park, NC

 
 
 

1. cable modem w/ISDN back-up connection: how to ?

Hi all.

We are installing a cable modem for Internet access on our LAN,
replacing our ISDN connection.  We have an NT tcp/ip network. We
currently use an Ascend router for ISDN.  I would like to have the LAN
connect to the cable modem, but use the ISDN router for a back-up, in
case of connection problems on cable.

Can anyone point me in the direction to set this up.  I know that I can
manually configure the ISDN router to change IP addresses on the LAN if
the cable connection fails, but I was thinking about an automated
solution that would access the LAN IP address of the NIC connected to
the cable modem first, and after a period of time, if no connection was
found, to route to the LAN IP address of the ISDN router.

Any suggestions will be welcomed.

John

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