" Does SLA work for Upper-Registration too?

Yes but with a caveat. OpenSBC has support for upper registration. This
is the term we coined to represent the ability of OpenSBC to hijack
registration bindings before it relays the registration to the actual
registrar. This allows OpenSBC to always stay in the middle of
registered UA and the SIP Server that needs far-end NAT traversal for
RTP.

The current implementation of Upper-Registration now also allows the
registrar to fork calls. To achieve this, OpenSBC not only hijacks the
registration by setting the contact address, it also now preserves the
original binding as an escaped parameter of the Contact-URI sent to the
upper registrar. If an upper registrar decides to fork a call to an SLA
account, OpenSBC would be able to identify the carrect binding to be
used for each forked transactions.

It is worth noting, however, that OpenSBC only has partial support for
Merged Requests" Merge Request is the term used for the situation
whereby INVITEs that are forked arrives at the same server. Since
OpenSBC is a B2BUA rather than a SIP Proxy, it can only reserve one RTP
session for each call. This results to an undesirable complication in
upper registrar forking where only the first INVITE received by OpenSBC
gets a B2BUA session. The rest of the merge requests are simply relayed
by OpenSBC. Yes, the serial fork would still ring the multiple devices
but only the first INVITE will have the ability to anchor media. This
is a deal breaker if the mutiple devices are behind a NAT. As of this
moment, at least until a solution is found to this problem, I do not
recommend upper registrar to fork calls towards OpenSBC."

How do I do this? What do I have to change in my upper registration Route List.

Thank you,

Oliver