I have a routing conundrum that I can't quite figure out.
I am designing a hub and spoke network based on my customer's specification and purchased equipment.
They want to implement dual site redundancy and have therefore purchased 2 Frame-relay routers and 2 ISDN routers for their data-centres (Production and DR). Each spoke site will access the data-centres in the following order
1st choice - Production data-centre frame-relay router
2nd choice - DR data-centre frame-relay router
3rd choice - Production data-centre ISDN router
4th choice - DR data-centre ISDN router.
The data-centres are part of the same OSPF area (0), each spoke site will sit in its own stub OSPF area and so the routing over the frame-relay links is easy - both hub routers advertise a default route, the DR link with a higher cost and so less preferred, the spoke router advertises its networks the same way over both links, again with the DR link having a higher ospf cost and so less preferred. Simple.
For the ISDN backup, I have a floating default route that won't kick in unless both default routes learnt over the frame-links are retracted, again simple stuff. The issue I have is how to get the 4th choice ISDN router to work in the unlikely event that it's ever needed. The reason for its existence is that there are 40 spoke sites and if all were to send or receive traffic simultaneously then both hub site ISDN PRIs would be needed as they are ISDN 20s (20B+D) and thus the production hub site link would be unable to accept a 21st call, hence the need for the 2nd PRI.
I know I can configure 2 dialer strings on the spoke, the 2nd one being used if the first call fails, good so far, but I can't work out how to get the return traffic to route back over the 2nd ISDN router. The hub routers obviously need to advertise the spoke source subnet in to the core, I can't think of a way of the DR ISDN router only advertising itself as the next hop for the spoke if a call came in to it, but otherwise the Production ISDN router advertises the route (with a high admin distance so the Frame-relay link routes are chosen if available).
In terms of hub to spoke traffic flows in this scenario I just don't think there's a way for the network to "know" that the production ISDN link is busy and re-route via the DR link. If anybody think otherwise, please let me know!
Apologies if this is long-winded, any ideas whatsoever would be incredibly appreciated