I have a customer who has two mirror sites at different physical locations. They have a primary site and a backup site along with 6 remote sites. At the primary and backup site they have 2 366x's one for primary WAN links and the other for the ISDN PRI's they already have. They are using EIGRP. The requirements are to dial the primary site and use the secondary only if the primary is unreachable. What is the best way to accomplish this goal between using dialer-watch, floating statics, etc...I have done this before but would like some guidance when having two ISDN backup sites-
You can add multiple dialer strings or dialer maps one after the other. ALways the router would dial the primary map (the first map it sees on the interface). If that fails or it gets a busy tone, it would switch over to the second dialer map (string). So this would give you good backup.
You have a range of choices, depending upon the degree of sophistication available for design and management of the network, the cost of downtime and the incremental cost of using the secondary location when the primary is available.
I usually use Shanky's approach if I can because it is the simplest. But be aware that once the secondary comes up, no further calls to the primary will be attempted which may make it unsuitable for you. It can also fail under conditions where the call succeeds but routing or authentication fails, a mode of failure which _should_ not occur, but take my word for it, unless you have perfect configuration management, plan on it!
True dial backup for dial backup (assuming the secondary should only be used when the primary is unavailable) is not particularly difficult in an EIGRP based network. I've been doing it for years (see the white paper "Using BGP to Trigger Multiple Levels of Dial Backup on Cisco Routers" on my web site). You can use the same addressing tricks with dialer watch and avoid the use of BGP, just make sure you are using at least IOS 12.1(7) to get a relatively bug free implementation of dialer watch. I also talk about these and other dial backup issues in my book, more details on my web site.
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