[cisco-nas] PPP dialer -- "failover" routing

[Roth, Kevin P.]

I have another PPP Dialer question, for Cisco 3845's with PVDM2 digital
modems.

I have a primary and a backup operations center (POC and BOC), each of
which has a 3845 dial-on-demand router.  Each OC will have its own set
of four T-1 PRI lines and sufficient PVDM2 cards to handle up to 92
simultaneously active PPP dialer connections.  But occasionally we'll
want to have more than 92 active connections originating from a client
machine at a single OC.  We can safely assume the other OC will be idle
at that time, so we'll have combined T-1 capacity for 184 connections,
but I need help to find a way to setup routing to be able to "failover"
to the other OC's 3845 as needed.

We would like the dialer map ip's on each 3845 to be identical.  If we
want to call out to remote site "X", we want to be able to open an IP
connection to IP address "10.1.2.3" and let the network handle all of
the routing details.  We do not want to have to maintain two IP address
ranges for each remote site, and we do not want our client machines to
have to know how to try an alternate IP address if the first one fails
to get through.

We do have network connectivity between the two OC's.  And we have some
cost-based routing in place so that client machines at the POC will be
routed to the POC 3845, and vice-versa.  The missing link is how to
force a connection to route to the "other" OC if there's an
out-of-capacity condition on the local T-1's.

Here's an example:  

We have a client machine at the POC which is originating 93+
dialer-mapped IP connections to 93+ remote sites.  We want the 3845 at
the POC to dial-up the first 92 PPP connections, until all of the POC's
T-1 channels are in use.  For the 93rd connection, we want a way for the
BOC's T-1 channels to be used, which is completely transparent to the
originating client machine.  Could the traffic be "re-routed" somehow to
the BOC's 3845, where the BOC 3845 can dial-up the PPP connection?  Or
is there a way for the POC's 3845 to "reach across the network" and
utilize the BOC's T-1 channels?  This situation would be reversed if the
originating client machine is at the BOC...

Thanks for any insight you guys can give me!
~ Kevin
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