[Serusers] ACK before INVITE on re-INVITE causes Cisco AS's to return a 500 error

Hey all, This has more to do with Asterisk & Cisco AS's IMO but I thought it would be interesting to get some responses from this group before I begin the battle with Cisco to get a bug report underway. We have an Asterisk box acting as a PBX sending calls to our SER instances. The asterisk box has "reinvite=yes" in the sip.conf file which causes Asterisk to send a re-INVITE for the call once it receives the 200 OK. This re-INVITE has it's SDP changed to that of the IP Phone behind the Asterisk box so that all media doesn't need to traverse it. This works fine except in certain instances where messages get out of order and it causes our Cisco GW's to error the call with a generic 500 message. When the messages are in the correct order the 500 error does not appear. So here's the situation, user behind the asterisk box picks up the phone and dials a PSTN destination. Asterisk fires off the initial invite and everything proceeds as normal and we eventually receive the 200 OK message to which the Asterisk box replies with ACK and a new INVITE that come in this order (X = Asterisk, Y = SER, Z = Cisco): U 2005/11/15 16:51:51.557322 X.X.X.X:5060 -> Y.Y.Y.Y:5060 ACK # U 2005/11/15 16:51:51.557948 X.X.X.X:5060 -> Y.Y.Y.Y:5060 INVITE SER accordingly processes the two messages and replies 100 Trying to the new INVITE message. Here's where the problem occurs, SER then sends these messages out to the PSTN gateway but the order has been reversed: # U 2005/11/15 16:51:51.558937 Y.Y.Y.Y:5060 -> X.X.X.X:5060 SIP/2.0 100 trying # U 2005/11/15 16:51:51.559000 Y.Y.Y.Y:5060 -> Z.Z.Z.Z:5060 INVITE # U 2005/11/15 16:51:51.559657 Y.Y.Y.Y:5060 -> X.X.X.X:5060 ACK When this happens the Cisco GW replies with a 500 error message and the following is sent to the Cisco's logs: # U 2005/11/15 16:51:51.711355 Z.Z.Z.Z:5060 -> Y.Y.Y.Y:5060 SIP/2.0 500 Server Internal Error. Nov 15 21:51:51.613: HandleUdpSocketReads :Msg enqueued for SPI with IPaddr: Y.Y.Y.Y:5060 Nov 15 21:51:51.613: *****CCB found in UAS Request table. ccb=0x653BCD0C Nov 15 21:51:51.613: CCSIP-SPI-CONTROL: act_sentsucc_new_message Nov 15 21:51:51.617: CCSIP-SPI-CONTROL: act_sentsucc_new_message_request Nov 15 21:51:51.617: Queued event from SIP SPI : SIPSPI_EV_SEND_MESSAGE Nov 15 21:51:51.617: sip_stats_status_code Nov 15 21:51:51.617: sipSPICheckRequest: CheckRequest fail on method 102 error code: 2 and status: 500 Is there any parameter that I can set to ensure that message leave SER in the order they enter? Should the Cisco be able to handle this condition (I check a number of different sections of the RFC but couldn't find anything specific for either side)? Thanks, Evan