ISDN Summary ON CCIELAB

First, most ISDN task is asked for back-up link to create a full reachability to stub router in case of F/R link failure.
This keyword is to restore full reachability. 

It is only possible by
1) two floating static routes ( default route from stub router to the rest of network and a specific static route from the other router to the stub router),
2) ip ospf demand circuit or Rip tigger w/ more metric ,
3) Dynamic routing protocol w/ dialer watch group and supressing routing protocol over ISDN,
4)back-up link
5) Rip w/ Snapshot routing.

1)Two floating static route is very straight forward and interesting traffic could be all ip packet. All IP traffic can be interesting traffic

2-1) ip ospf demand circuit is also straight forward and "no peer neighbor.." is important to prevent link flapping.The dynamic link cost needs to be something high to play a similar role as floting route with higher AD. I don't think ip OSPF cost need any value .  I think OSPF Demand link has a high cost by default. All IP traffic can be interesting traffic.

2-2) Rip trigger will require higher metric ( off-set). Otherwise, traffic may continue to flow over ISDN after F/R is restored. All IP traffic can be interesting traffic.  To function as back-up link, Rip need to run over ISDN as trigger mode and F/R as any mode.

3) If Dialer watch group is configured to bring up ISDN link, an IGP protocol should run across ISDN link to reslove full reachability.
It is because dialer watch group doesn't create any route itself in to route look-up table. But what is important is to make this IGP protocol hello or update not interesting traffic.  This will make normall case, igp peering down, but when dialer watch group trigger, igp will be able to send hello so that igp will be able to update routing table, which will create full reachability. If IGP is OSPD, then ip ospf demand circuit will cover most case of back-up.  But dialer watch group can be used to meet a specific task such as dial ISDN if Vlan X dispear from routing table even if  frame-relay is up, or ISDN should be up until F/R is up or until VLAN X show up again on ISDN.  Ip OSPF demand will can make ISDN up when there is topological change but as long as topology is stable even with F/R down and if there is no interesting traffic, the ISDN link will eventually go down.   Also some other advantage of dialer watch is that it doesn't care of topologic change while IP ospf demand or rip tigger will dial isdn when there is topological changes that have nothing to do with F/R link.

4) Back-up link is  much simpler than dialer watch.  It doesn't have to supress IGP hello.  It doesn't need high IGP link cost.  For this reason,most task will ask not to use back-up link, which is the problem.

5) Snapshot routing is basically used to supress rip route update over ISDN. With higher rip metric over ISDN, it can function as back-up link if rip runs over f/r link and ISDN.