Recently at my company I was put in charge of configuring and deploying Cisco 881 routers and creating a Site-to-Site VPN's back to our ASA at the corporate office. I think this might have something to do with my co-workers knowing that I'm studying my CCNA: Security, oh well I didn't mind at all and kinda volunteered for this project. Well I wanted to learn about Site-to-Site VPN's through my IINS book before I started this project but the deployment happened before I could get to Chapter 4 and I'm really OCD about skipping chapters. In the end it worked out well because I think reading the chapter on Site-to-Site VPN's before ever configuring one beforehand would of just confused me.
I was able to open a TAC case to have an engineer help me get the bare bones configuration up. I then created a template and tweaked it to the point were all you have to do is change the IP's, allow the IP's on the ASA ACL, and create a static route and you were good to go. I was able to configure a group of 5 Cisco 881's in about 2 hours taking about 20 minutes each, the longest time was spent taking the packaging off of the router. The configuration took about 5 minutes.
This weekend I spent some time setting up a site-to-site configuration from scratch. For what ever reason I could never get the tunnel up and I quadruple checked the configuration including starting the lab over from scratch! It wasn't until I found some documentation that I realized why the tunnel never attempted to be created. The tunnel was only created when "interesting' traffic was sent to the other peer that's involved in the VPN process. I did a simple ping from end host to the other and just like magic the tunnels came right up!
A quick show crypto isakmp sa will show you rather your tunnel is up and alive: