I am attempting to setup a Cisco 851 in a remote office to communicate with an 871 in the main office. The primary purpose is to allow VoIP using our PBX, with access to our file server as a secondary requirement. I currently have both units and the VoIP phone sitting on my desk connected in the simplest configuration I could come up with, as follows:

VoIP Phone - 851 - 871 - switch - PBX

The 851 and 871 are connected by their WAN ports. I can get the units to communicate just fine, and have been able to setup either a GRE/IP or IP/IP tunnel between the two units and the VoIP phone can link to the PBX and functions great. However, when I try to establish a VPN connection between the two units using the Cisco SDM utility, a link is established but performance is terrible. When pinging across the VPN, I have anywhere from 50-100% packet loss (but the delay is still fine at 1-3ms). Everything appears to be up from a link standpoint, but packets seem to just randomly dissapear. The only network traffic is from the VoIP phone and there is practically no load on the connection according to either router. The VoIP phone can eventually establish a connection to the PBX, but the phone is unusable.

My ability to get a simple tunnel to work fine leads me to believe this isn't a hardware problem. I am sure I am overlooking something simple but have been beating my head against the wall for a week now. Any assistance will be greatly appreciated.

Thanks,

Steve