Quick question....can you set up HSRP on a WAN interface, one frame relay and the other ISDN line or is this for only ethernet?
HSRP works on LAN/Ethernet interfaces. It does not work on Frame Relay or ISDN interfaces.
just to make things clear for you. This is scenario where you would use HSRP/VRRP/GLBP. The idea behind these protocols is the same but the way in which they function is slightly different. But don't become confused by that!
Look at this scenario. There are 2 routers on the same network and they are interconnected through switch. Hosts (PCs, Laptops) are connected through switch as well. To access the Internet, they have to have some default gateway ( i.e. Router). But which router they should use? There are two of them!
If you manually configure hosts to use only Router 1, Router 2 will be unused. And what if Router 1 fails? Router 2 could be backup because he also knows how to get to the internet, but how? Well, this is the time when HSRP/VRRP/GLBP (FHRP protocols) comes to play.
One more thing. The family of these protocols is called FHRP (First hop redundancy protocol) and that should be enough to understand their purpose. They simply provide redundancy of first hop from the view of host - therefore they provide redundancy of default gateway.
When you configure these two Routers with any of these FHRP they will use ONE virtual IP address which you configure hosts to use and when one of the Router fails, the other will forward the traffic. In addition - you can configure it in such manner that Routers will keep an eye on the serial interface as well (track interface command) and when it comes down, the other Router will forward the traffic. This is handy because otherwise the FHRP would work only when the whole Router (or fa0/0) comes down. It would not detect any failure, hosts would send packet to this Router and Router, which serial interface is down, would drop the packets.
I hope that will make it more clear to you. So you only configure redundancy for hosts IN THE SAME NETWORK to the routers IN THE SAME NETWORK - 192.168.0.0/24 in my example. You CANNOT configure redundany like this between DIFFFERENT NETWORKS. The concept of FHRP is not designed to solve these kind of problems...