I am a bit confused. I suspect with SBC you mean the Kamailio proxy?

On 05.02.2014 14:54, Diego Alejandro Ozuna Escalada wrote:

When a UA sends an INVITE to the SBC, the responses (back) to UA are
being sent with the correct external source IP (X.X.248.194) but if
there are retransmissions of these responses, after 6 seconds, the
source IP changes to the internal ip (X.X.248.21)

Are you sure it is a retransmission, or maybe it is a new branch of an existing transaction, e.g. triggered by a failure route? You can differ them by different branch id in the topmost Via header (carefully check the last character). If it is really due to a new branch created in a failure route, make sure to force the send socket also in the failure route.

PS: If you have a nice working IP setup, e.g. one external interface (used by the default route) and one internal interface (a different subnet, dedicated routes for the internal network) then you could just set mhomed=1 in kamailio.cfg and Kamailio can automatically always select the correct send-socket.

regards
Klaus

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