Calling cisco IOS gurus..

I've got 3 cisco routers participating in a DMVPN (2x 857, 1x 877), which works well.

Enabling CDP on the 857's works as expected, but enabling it on the 877 ends up killing the VPN to/from the 877.

Any pointers on where to look?

877:

System image file is "flash:c870-advipservicesk9-mz.124-24.T.bin"

857's:

System image file is "flash:c850-advsecurityk9-mz.124-15.T15.bin"