processing of Informational mode failed??

[admin_2]

I'm trying to build an IPSec tunnel between a 2600 router and a PIX 520 firewall, the config is the same as the example

http://www.cisco.com/warp/public/110/39.html

except I've not included the NAT bits.

When I try to pass traffic, I get the following message out from 'debug isakmp'

CRYPTO-6-IKMP_MODE_FAILURE: processing of Informational mode failed with peer xxx.xxx.xxx.xxx

but I can't find anywhere on the Cisco site (indeed, on the Internet!) what this means, or how I can fix it.

[#2]

It means that One side has a certificate and the other does not.